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>