libxfs: Optimize the loop for xfs_bitmap_empty

If there is any non zero bit in a long bitmap, it can jump out of the
loop and finish the function as soon as possible.

Signed-off-by: Jia He <hejianet@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>