xfs: fix remote xattr valuelblk check

xfs: fix remote xattr valuelblk check

In debugging other problems with generic/753, it turns out that it's
possible for the system go to down in the middle of a remote xattr set
operation such that the leaf block entry is marked incomplete and
valueblk is set to zero.  Make this no longer a failure.

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>