hang in vcache_vget()

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

Hello

I experienced a system freeze on NetBSD-10.0/i386. Many processes 
waiting on tstile, and one waiting on vnode, with this backtrace:
sleepq_block
cv_wait
vcache_vget
vcache_get
ufs_lookup
VOP_LOOKUP
lookup_once
namei_tryemulroot.constprop.0
namei
vn_open
do_open
do_sys_openat

I regret I did not took the time to show vnode. 

Is it worth a PR? I have no clue if it can be reproduced.
-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost