Add vfs_hash_ref(9) function, which finds a vnode by the hash value
and returns it referenced.

The function is similar to vfs_hash_get(9), but unlike the later,
returned vnode is not locked. This operation cannot be requested with
the vget(9) flags.

Reviewed and tested by: rmacklem
Sponsored by: The FreeBSD Foundation
MFC after: 1 week