xfs: keep quota directory inode loaded

In the same vein as the previous patch, there's no point in the metapath
scrub setup function doing a lookup on the quota metadir just so it can
validate that lookups work correctly.  Instead, retain the quota
directory inode in memory for the lifetime of the mount so that we can
check this meaningfully.

Cc: <stable@vger.kernel.org> # v6.13-rc1
Fixes: 128a0552 ("xfs: scrub quota file metapaths")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>