xfs: improve the xg_active_ref check in xfs_group_free

xfs: improve the xg_active_ref check in xfs_group_free

Split up the XFS_IS_CORRUPT statement so that it immediately shows
if the reference counter overflowed or underflowed.

I ran into this quite a bit when developing the zoned allocator, and had
to reapply the patch for some work recently.  We might as well just apply
it upstream given that freeing group is far removed from performance
critical code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>