xfs: open code insert range extent split helper

The insert range operation currently splits the extent at the target
offset in a separate transaction and lock cycle from the one that
shifts extents. In preparation for reworking insert range into an
atomic operation, lift the code into the caller so it can be easily
condensed to a single rolling transaction and lock cycle and
eliminate the helper. No functional changes.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-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>