xfs: factor agf counter updates into a helper

Updating the AGF and transactions counters is duplicated between allocating
and freeing extents.  Factor the code into a common helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>