NFSv4, NFSD: move enum nfs_cb_opnum4 to include/linux/nfs4.h

Callback operations enum is defined in client and server, move it to
common header file.

Signed-off-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Acked-by: Anna Schumaker <Anna.Schumaker@netapp.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>