xfs: add a per-leaf block callback to xchk_xattr_walk

Add a second callback function to xchk_xattr_walk so that we can do
something in between attr leaf blocks.  This will be used by the next
patch to see if we should flush cached parent pointer updates to
constrain memory usage.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>