uma_zalloc_debug warnings after r362231

I've started getting the following after r362231 (I think) when plugging 
USB drive, not seeing any real issues though:

ugen0.9: <Western Digital Elements 25A2> at usbus0 

umass0 numa-domain 0 on uhub4 

umass0: <Western Digital Elements 25A2, class 0/0, rev 2.10/10.26, addr 
10> on usbus0
umass0:  SCSI over Bulk-Only; quirks = 0xc101
umass0:2:0: Attached to scbus2
uma_zalloc_debug: zone "kenv" with the following non-sleepable locks held:
exclusive sleep mutex CAM device lock (CAM device lock) r = 0 
(0xfffff8000dcb6cd0) locked _at_ /usr/src/sys/cam/scsi/scsi_pass.c:674
stack backtrace:
#0 0xffffffff80c32981 at witness_debugger+0x71
#1 0xffffffff80c3391d at witness_warn+0x40d
#2 0xffffffff80efe426 at uma_zalloc_arg+0x46
#3 0xffffffff80b741da at getenv_string_buffer+0x3a
#4 0xffffffff80b74907 at getenv_quad+0x17
#5 0xffffffff80b748d2 at getenv_int+0x12
#6 0xffffffff803a7f1a at daregister+0x1ea
#7 0xffffffff8037445b at cam_periph_alloc+0x57b
#8 0xffffffff803a7872 at daasync+0x2c2
#9 0xffffffff80381aa2 at xpt_async_process_dev+0x152
#10 0xffffffff8037d682 at xpt_async_process+0x312
#11 0xffffffff8037ddf2 at xpt_done_process+0x382
#12 0xffffffff8037fdb5 at xpt_done_td+0xf5
#13 0xffffffff80b831e0 at fork_exit+0x80
#14 0xffffffff810413ce at fork_trampoline+0xe
da0 at umass-sim0 bus 0 scbus2 target 0 lun 0
da0: <WD Elements 25A2 1026> Fixed Direct Access SPC-4 SCSI device
da0: Serial Number 575845314136394150553136
da0: 40.000MB/s transfers
da0: 953837MB (1953458176 512 byte sectors)
da0: quirks=0x2<NO_6_BYTE>