Stack backtrace for drm and malloc(M_WAITOK)

With Jason's new malloc implementation, I had to install
xorg-server-snap to get a functioning x server on a amd64
system.  When the drm module is loaded, I'm see numerous
backtraces in /var/log/messages.

KDB: stack backtrace:
kdb_backtrace() at kdb_backtrace+0x37
witness_warn() at witness_warn+0x2c1
uma_zalloc_arg() at uma_zalloc_arg+0x69
malloc() at malloc+0xf5
sysctl_add_oid() at sysctl_add_oid+0xa9
alloc_bounce_zone() at alloc_bounce_zone+0x418
bus_dma_tag_create() at bus_dma_tag_create+0x1ea
drm_pci_alloc() at drm_pci_alloc+0xe6
drm_do_addbufs_pci() at drm_do_addbufs_pci+0x24b
drm_addbufs_pci() at drm_addbufs_pci+0xeb
drm_addbufs_ioctl() at drm_addbufs_ioctl+0x89
drm_ioctl() at drm_ioctl+0x35c
giant_ioctl() at giant_ioctl+0x68
devfs_ioctl_f() at devfs_ioctl_f+0xc9
ioctl() at ioctl+0x436
syscall() at syscall+0x350
Xfast_syscall() at Xfast_syscall+0xa8
--- syscall (54, FreeBSD ELF64, ioctl), rip = 0x200cce29c, \
   rsp = 0x7fffffffe868, rbp = 0x202bfa890 ---
malloc(M_WAITOK) of "16", forcing M_NOWAIT with the following non-sleepable \
   locks held:
exclusive sleep mutex drmdma r = 0 (0xffffff02323b0130) locked _at_ \
   /usr/src/sys/modules/drm/drm/../../../dev/drm/drm_bufs.c:846
-- 
Steve