xfs: create a predicate to verify per-AG extents

xfs: create a predicate to verify per-AG extents

Source kernel commit: b65e08f83b119ae9345ed23d4da357a72b3cb55c

Create a predicate function to verify that a given agbno/blockcount pair
fit entirely within a single allocation group and don't suffer
mathematical overflows.  Refactor the existng open-coded logic; we're
going to add more calls to this function in the next patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>