xfs: check pre-metadir fields correctly

The checks that were added to the superblock scrubber for metadata
directories aren't quite right -- the old inode pointers are now defined
to be zeroes until someone else reuses them.  Also consolidate the new
metadir field checks to one place; they were inexplicably scattered
around.

Cc: <stable@vger.kernel.org> # v6.13-rc1
Fixes: 28d756d4 ("xfs: update sb field checks when metadir is turned on")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>