fstests: test fix for an agbno overflow in __xfs_getfsmap_datadev

fstests: test fix for an agbno overflow in __xfs_getfsmap_datadev

Dave Chinner reported that xfs/273 fails if the AG size happens to be an
exact power of two.  I traced this to an agbno integer overflow when the
current GETFSMAP call is a continuation of a previous GETFSMAP call, and
the last record returned was non-shareable space at the end of an AG.

This is the regression test for that bug.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>