vfs_hash_insert: ensure that predicate is true

vfs_hash_insert: ensure that predicate is true

After vnode lock, recheck v_hash. When vfs_hash_insert() is used with
a predicate, recheck it after the selected vnode is locked. Since
vfs_hash_lock is dropped, vnode could be rehashed during the sleep for
the vnode lock, which could go unnoticed there.

Reported and tested by:	pho
Reviewed by:	mckusick, rmacklem
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D31464