Fix FFS panic that happens after the mounted device went away with
softupdates enabled. Backtrace would look like this:
#11 0xffffffff80a50063 in panic (fmt=0xffffffff81c6e7f0 <cnputs_mtx+24> "\004") at /usr/home/trasz/svn-ssh/android/sys/kern/kern_shutdown.c:710
#12 0xffffffff80d05dfe in softdep_deallocate_dependencies (bp=0xfffffe01ea78cc40) at /usr/home/trasz/svn-ssh/android/sys/ufs/ffs/ffs_softdep.c:14257
#13 0xffffffff80af662c in buf_deallocate (bp=0xfffffe01ea78cc40) at /usr/home/trasz/svn-ssh/android/sys/sys/buf.h:429
#14 brelse (bp=0xfffffe01ea78cc40) at /usr/home/trasz/svn-ssh/android/sys/kern/vfs_bio.c:2318
#15 0xffffffff80b14b57 in flushbuflist (bufv=<optimized out>, flags=<optimized out>, bo=<optimized out>, slpflag=<optimized out>, slptimeo=0)
at /usr/home/trasz/svn-ssh/android/sys/kern/vfs_subr.c:1779
#16 0xffffffff80b147af in bufobj_invalbuf (bo=0xfffff8007e334658, flags=0, slpflag=0, slptimeo=0) at /usr/home/trasz/svn-ssh/android/sys/kern/vfs_subr.c:1661
#17 0xffffffff80b17dde in vinvalbuf (vp=<optimized out>, flags=0, slpflag=0, slptimeo=0) at /usr/home/trasz/svn-ssh/android/sys/kern/vfs_subr.c:1721
#18 vgonel (vp=0xfffff8007e334588) at /usr/home/trasz/svn-ssh/android/sys/kern/vfs_subr.c:3353
#19 0xffffffff80b1838f in vgone (vp=0xfffff8007e334588) at /usr/home/trasz/svn-ssh/android/sys/kern/vfs_subr.c:3229
#20 0xffffffff8091c654 in devfs_delete (dm=0xfffff8000d71ad80, de=0xfffff80004565100, flags=<optimized out>) at /usr/home/trasz/svn-ssh/android/sys/fs/devfs/devfs_devs.c:393