xfs: don't rely on extent indices in xfs_bmap_collapse_extents

Rewrite xfs_bmap_collapse_extents so that we don't rely on extent indices
except for iterating over them.  Not being able to iterate to the next
extent is a sufficient exit condition, and we don't need to do any extent
count games given that:

  a) we already flushed all delalloc extents past our start offset
     before doing the operation
  b) xfs_iext_count() includes delalloc extents anyway

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>