xfs: split xfs_bmap_shift_extents

Have a separate helper for insert vs collapse, as this prepares us for
simplifying the code in the next patches.

Also changed the done output argument to a bool intead of int for both
new functions.

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>