NFSv4: Ensure delegation recall and byte range lock removal don't conflict

Add a mutex to the struct nfs4_state_owner to ensure that delegation
recall doesn't conflict with byte range lock removal.

Note that we nest the new mutex _outside_ the state manager reclaim
protection (nfsi->rwsem) in order to avoid deadlocks.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4state.c

@@ -519,6 +519,7 @@ nfs4_alloc_state_owner(struct nfs_server *server,
 	atomic_set(&sp->so_count, 1);
 	INIT_LIST_HEAD(&sp->so_lru);
 	seqcount_init(&sp->so_reclaim_seqcount);
+	mutex_init(&sp->so_delegreturn_mutex);
 	return sp;
 }