Every kernel I've built in the last few days panics in exactly this way,
after any io (sync/fsck/ls):

panic pool_get: must have NOWAIT

db>t
...
_panic
_pool_get
_uvm_map_clip_start
_uvm_km_free_wakeup
_pmap_remove_mapping
_pmap-remove
_uvm_unmap_remove
_uvm_km_free_wakeup
_pmap_release
_pmap_destroy
_uvm_spae_free
_cpu_exit
_exit1
_sys_exit
_syscall
_trap

The last kernel that works was built against August 30's sup.