NFS: Clean up nfs4_find_state_owners_locked()

There's no longer a need to check the so_server field in the state
owner, because nowadays the RB tree we search for state owners
contains owners for that only server.

Make nfs4_find_state_owners_locked() use the same tree searching logic
as nfs4_insert_state_owner_locked().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>