xfs: switch xfs_bmap_local_to_extents to use xfs_iext_insert

xfs: switch xfs_bmap_local_to_extents to use xfs_iext_insert

Source kernel commit: 50bb44c28614205def9e711190842b4c0242ae79

Use the helper instead of open coding it, to provide a better abstraction
for the scalable extent list work.  This also gets an additional assert
and trace point for free.

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>