xfs: move some code around inside xfs_bmap_shift_extents

xfs: move some code around inside xfs_bmap_shift_extents

Source kernel commit: 05b7c8ab2be71e6fef4615451e7af1bc79ffdf29

For the first right move we need to look up next_fsb.  That means
our last fsb that contains next_fsb must also be the current extent,
so take advantage of that by moving the code around a bit.

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>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>