xfs: fix the xattr scrub to detect freemap/entries array collisions

xfs: fix the xattr scrub to detect freemap/entries array collisions

In the previous patches, we observed that it's possible for there to be
freemap entries with zero size but a nonzero base.  This isn't an
inconsistency per se, but older kernels can get confused by this and
corrupt the block, leading to corruption.

If we see this, flag the xattr structure for optimization so that it
gets rebuilt.

Cc: <stable@vger.kernel.org> # v4.15
Fixes: 13791d3b833428 ("xfs: scrub extended attribute leaf space")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>