xfs: don't log the inode in xfs_fs_map_blocks if it

Even if we are asked for a write layout there is no point in logging
the inode unless we actually modified it in some way.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>