at SVN r347375, terminating/restarting openvpn on tap causes panic

Seems there's a lock or tuntap issue after recent changes. Restarting
openvpn (configured to use /dev/tap) yields a panic as follows:

(kgdb) bt
#0  doadump () at src/sys/amd64/include/pcpu.h:241
#1  0xffffffff808393c8 in kern_reboot (howto=260) at
/usr/src/sys/kern/kern_shutdown.c:470
#2  0xffffffff80839826 in vpanic (fmt=<value optimized out>, ap=<value
optimized out>) at /usr/src/sys/kern/kern_shutdown.c:896
#3  0xffffffff80839663 in panic (fmt=<value optimized out>) at
/usr/src/sys/kern/kern_shutdown.c:823
#4  0xffffffff80c31686 in trap_fatal (frame=0xfffffe0072b14510, eva=156)
at /usr/src/sys/amd64/amd64/trap.c:946
#5  0xffffffff80c31a39 in trap_pfault (frame=0xfffffe0072b14510,
usermode=0) at src/sys/amd64/include/cpufunc.h:423
#6  0xffffffff80c30dbe in trap (frame=0xfffffe0072b14510) at
/usr/src/sys/amd64/amd64/trap.c:441
#7  0xffffffff80c0d295 in calltrap () at
/usr/src/sys/amd64/amd64/exception.S:232
#8  0xffffffff80a15387 in ip_output (m=<value optimized out>, opt=<value
optimized out>, ro=0x0, flags=0, imo=0xfffffe0072b14780, inp=0x0) at
/usr/src/sys/netinet/ip_output.c:362
#9  0xffffffff809ffeb4 in igmp_intr (m=<value optimized out>) at
/usr/src/sys/netinet/igmp.c:3455
#10 0xffffffff80975a1f in netisr_dispatch_src (proto=2, source=<value
optimized out>, m=<value optimized out>) at /usr/src/sys/net/netisr.c:1122
#11 0xffffffff809fe08a in igmp_fasttimo () at
/usr/src/sys/netinet/igmp.c:496
#12 0xffffffff808c5854 in pffasttimo (arg=<value optimized out>) at
/usr/src/sys/kern/uipc_domain.c:521
#13 0xffffffff80853df3 in softclock_call_cc (c=0xffffffff813f7f48,
cc=0xffffffff814c9ac0, direct=0) at /usr/src/sys/kern/kern_timeout.c:731
#14 0xffffffff808542b9 in softclock (arg=0xffffffff814c9ac0) at
/usr/src/sys/kern/kern_timeout.c:869
#15 0xffffffff807fd0c4 in ithread_loop (arg=<value optimized out>) at
/usr/src/sys/kern/kern_intr.c:1129
#16 0xffffffff807f9f33 in fork_exit (callout=0xffffffff807fcef0
<ithread_loop>, arg=0xfffff80002620100, frame=0xfffffe0072b14ac0) at
/usr/src/sys/kern/kern_fork.c:1058
#17 0xffffffff80c0e28e in fork_trampoline () at
/usr/src/sys/amd64/amd64/exception.S:995
#18 0x0000000000000000 in ?? ()

	imb