xfs: use xfs_iext_*_extent helpers in xfs_bmap_shift_extents

This abstracts the function away from details of the low-level extent
list implementation.

Note that it seems like the previous implementation of rmap for
the merge case was completely broken, but it no seems appear to
trigger that.
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>