xfs: adjust min_block usage in xfs_verify_agbno

xfs: adjust min_block usage in xfs_verify_agbno

There's some weird logic in xfs_verify_agbno -- min_block ought to be
the first agblock number in the AG that can be used by non-static
metadata.  However, we initialize it to the last agblock of the static
metadata, which works due to the <= check, even though this isn't
technically correct.

Change the check to < and set min_block to the next agblock past the
static metadata.  This hasn't been an issue up to now, but we're going
to move these things into the generic group struct, and this will cause
problems with rtgroups, where min_block can be zero for an rtgroup that
doesn't have a rt superblock.

Note that there's no user-visible impact with the old logic, so this
isn't a bug fix.

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