xfs: mark nonzero sb_gquotino as corrupt on metadir filesystems

On a metadir filesystem, the superblock gquotino field is always zero
because we find the quota inodes through the metadata directory tree.
A nonzero value is therefore a corruption (as pointed out by LOLLM) so
mark the failure as such.

Cc: <stable@vger.kernel.org> # v6.13
Fixes: 06b20ef09ba163 ("xfs: check pre-metadir fields correctly")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>