Equivalent to xfs_ilock_data_map_shared, except for the attribute fork.

Make xfs_getbmap use it if called for the attribute fork instead of
xfs_ilock_data_map_shared.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NBen Myers <bpm@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>