xfs: move xattr scrub buffer allocation to top level function

Move the xchk_setup_xattr_buf call from xchk_xattr_block to xchk_xattr,
since we only need to set up the leaf block bitmaps once.

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