Assert this.
In the reported panic, vdestroy() fired the assertion "vp has namecache
for ..", because pseudofs may end up doing cache_enter() with reclaimed
dvp, after dotdot lookup temporary unlocked dvp.
Similar problem exists in ufs_lookup() for "." lookup, when vnode
lock needs to be upgraded.
Verify that dvp is not reclaimed before calling cache_enter().
Reported and tested by: pho
Reviewed by: kan
MFC after: 2 weeks
d5b92466a9