armv7 BETA3 panics when usb-disk inserted

With the 12.0-BETA3 BEAGLEBONE image, I very often see this panic
when I plug a USB attached SSD disk in.

	login: ugen1.2: <Seagate USB 2.0 Cable> at usbus1
	umass0 on uhub0
	umass0: <Seagate USB 2.0 Cable, class 0/0, rev 2.00/1.48, addr 2> on usbus1
	umass0:  SCSI over Bulk-Only; quirks = 0x8100
	umass0:0:0: Attached to scbus0
	da0 at umass-sim0 bus 0 scbus0 target 0 lun 0
	da0: <Seagate USB 2.0 Cable 0148> Fixed Direct Access SPC-2 SCSI device
	da0: Serial Number 2HC015KJ
	da0: 40.000MB/s transfers
	da0: 38166MB (78165359 512 byte sectors)
	da0: quirks=0x2<NO_6_BYTE>
	panic: _mtx_lock_sleep: recursed on non-recursive mutex CAM device lock _at_ /usr/src/sys/cam/scsi/scsi_da.c:2123

	cpuid = 0
	time = 1541273846
	KDB: stack backtrace:
	db_trace_self() at db_trace_self
		 pc = 0xc05c93f4  lr = 0xc0075dd8 (db_trace_self_wrapper+0x30)
		 sp = 0xc35dca40  fp = 0xc35dcb58
	db_trace_self_wrapper() at db_trace_self_wrapper+0x30
		 pc = 0xc0075dd8  lr = 0xc029d624 (vpanic+0x16c)
		 sp = 0xc35dcb60  fp = 0xc35dcb80
		 r4 = 0x00000100  r5 = 0x00000001
		 r6 = 0xc06d2cde  r7 = 0xc0a94fd8
	vpanic() at vpanic+0x16c
		 pc = 0xc029d624  lr = 0xc029d404 (doadump)
		 sp = 0xc35dcb88  fp = 0xc35dcb8c
		 r4 = 0x00000000  r5 = 0xd1eb1474
		 r6 = 0xc06ff75f  r7 = 0xc259b780
		 r8 = 0xd1eb1464  r9 = 0xc259b780
		r10 = 0x0000084b
	doadump() at doadump
		 pc = 0xc029d404  lr = 0xc0282c14 (__mtx_unlock_flags)
		 sp = 0xc35dcb94  fp = 0xc35dcbf0
		 r4 = 0xc029d404  r5 = 0xc35dcb94
	__mtx_unlock_flags() at __mtx_unlock_flags
		 pc = 0xc0282c14  lr = 0xc0282538 (__mtx_lock_flags+0xec)
		 sp = 0xc35dcbf8  fp = 0xc35dcc20
		 r4 = 0x00000000  r5 = 0xd1eb1464
		 r6 = 0xc06ff75f r10 = 0x0000084b
	__mtx_lock_flags() at __mtx_lock_flags+0xec
		 pc = 0xc0282538  lr = 0xc002f384 (daasync+0x5c)
		 sp = 0xc35dcc28  fp = 0xc35dcc70
		 r4 = 0xc0018574  r5 = 0xd375f940
		 r6 = 0x00000400  r7 = 0xc23ed900
		 r8 = 0x00000000  r9 = 0xc072ee95
		r10 = 0xd375f940
	daasync() at daasync+0x5c
		 pc = 0xc002f384  lr = 0xc000f6e4 (xpt_async_process_dev+0x220)
		 sp = 0xc35dcc78  fp = 0xc35dcca8
		 r4 = 0xc0018574  r5 = 0xd375f940
		 r6 = 0x00000400  r7 = 0xc002f328
		 r8 = 0xc2322320  r9 = 0xc072ee95
		r10 = 0xc2322300
	xpt_async_process_dev() at xpt_async_process_dev+0x220
		 pc = 0xc000f6e4  lr = 0xc000e614 (xptdevicetraverse+0xa4)
		 sp = 0xc35dccb0  fp = 0xc35dccd0
		 r4 = 0xd376994c  r5 = 0xd1eb1474
		 r6 = 0xc072ee95  r7 = 0xd1eb1000
		 r8 = 0xd3769900  r9 = 0xd41a2800
		r10 = 0xc000f4c4
	xptdevicetraverse() at xptdevicetraverse+0xa4
		 pc = 0xc000e614  lr = 0xc000e3a0 (xpttargettraverse+0x7c)
		 sp = 0xc35dccd8  fp = 0xc35dccf8
		 r4 = 0xd3769900  r5 = 0xd376994c
		 r6 = 0xd3769800  r7 = 0xc091a140
		 r8 = 0xd41a2800  r9 = 0xc000f458
		r10 = 0xd375f940
	xpttargettraverse() at xpttargettraverse+0x7c
		 pc = 0xc000e3a0  lr = 0xc000b3f4 ($a.10+0x148)
		 sp = 0xc35dcd00  fp = 0xc35dcdc0
		 r4 = 0x00000000  r5 = 0x00000400
		 r6 = 0xd3769900  r7 = 0xc091a140
		 r8 = 0xd41a2800  r9 = 0xd375f944
		r10 = 0xd375f940
	$a.10() at $a.10+0x148
		 pc = 0xc000b3f4  lr = 0xc000bbe8 (xpt_done_process+0x3c4)
		 sp = 0xc35dcdc8  fp = 0xc35dcdd8
		 r4 = 0xd41a2800  r5 = 0xc258ca80
		 r6 = 0x00000000  r7 = 0xc091a140
		 r8 = 0x00000001  r9 = 0x00000100
		r10 = 0xc35dcdfc
	xpt_done_process() at xpt_done_process+0x3c4
		 pc = 0xc000bbe8  lr = 0xc000dac4 (xpt_done_td+0xec)
		 sp = 0xc35dcde0  fp = 0xc35dce20
		 r4 = 0xc091a100  r5 = 0xc06d60c2
		 r6 = 0x00000000  r7 = 0xc091a140
	xpt_done_td() at xpt_done_td+0xec
		 pc = 0xc000dac4  lr = 0xc0262f88 (fork_exit+0xa0)
		 sp = 0xc35dce28  fp = 0xc35dce40
		 r4 = 0xc259b780  r5 = 0xc23f7390
		 r6 = 0xc000d9d8  r7 = 0xc091a100
		 r8 = 0xc35dce48  r9 = 0x00000000
		r10 = 0x00000000
	fork_exit() at fork_exit+0xa0
		 pc = 0xc0262f88  lr = 0xc05cbcd4 (swi_exit)
		 sp = 0xc35dce48  fp = 0x00000000
		 r4 = 0xc000d9d8  r5 = 0xc091a100
		 r6 = 0x00000000  r7 = 0x00000000
		 r8 = 0x00000000 r10 = 0x00000000
	swi_exit() at swi_exit
		 pc = 0xc05cbcd4  lr = 0xc05cbcd4 (swi_exit)
		 sp = 0xc35dce48  fp = 0x00000000


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