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>