xfs: only allocate free space bitmap for xattr scrub if needed

The free space bitmap is only required if we're going to check the
bestfree space at the end of an xattr leaf block.  Therefore, we can
reduce the memory requirements of this scrubber if we can determine that
the xattr is in short format.

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