xfs: move xfs_iext_insert tracepoint to report useful information

Move the tracepoint in xfs_iext_insert to after the point where we've
inserted the extent because otherwise we report stale extent data in
the ftrace output.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>