xfs: don't crash on corrupt /quotas dirent

If the /quotas dirent points to an inode but the inode isn't loadable
(and hence mkdir returns -EEXIST), don't crash, just bail out.

Cc: <stable@vger.kernel.org> # v6.13-rc1
Fixes: e80fbe1a ("xfs: use metadir for quota inodes")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>