vfs_hash_rehash(): require the vnode to be exclusively locked
Rehash updates v_hash. Also, rehash moves the vnode to different hash
bucket, which should be noticed in vfs_hash_get() after sleeping for
the vnode lock.
Reviewed by: mckusick, rmacklem
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31464