xfs: don't load local xattr values during scrub

Local extended attributes store their values within the same leaf block.
There's no header for the values themselves, nor are they separately
checksummed.  Hence we can save a bit of time in the attr scrubber by
not wasting time retrieving the values.

Regrettably, shortform attributes do not set XFS_ATTR_LOCAL so this
offers us no advantage there, but at least there are very few attrs in
that case.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>