nfsd: use correct enum type in decode_cb_op_status

Use enum nfs_cb_opnum4 in decode_cb_op_status. This fixes warnings
seen with clang:
  fs/nfsd/nfs4callback.c:451:36: warning: implicit conversion from
      enumeration type 'enum nfs_cb_opnum4' to different enumeration
      type 'enum nfs_opnum4' [-Wenum-conversion]
        status = decode_cb_op_status(xdr, OP_CB_SEQUENCE, &cb->cb_seq_status);
                 ~~~~~~~~~~~~~~~~~~~      ^~~~~~~~~~~~~~
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -223,8 +223,8 @@ static int nfs_cb_stat_to_errno(int status)
 	return -status;
 }
 
-static int decode_cb_op_status(struct xdr_stream *xdr, enum nfs_opnum4 expected,
-			       int *status)
+static int decode_cb_op_status(struct xdr_stream *xdr,
+			       enum nfs_cb_opnum4 expected, int *status)
 {
 	__be32 *p;
 	u32 op;