Instead of dividing by 8 and then performing log2 by hand, use a more
readable calculation.
The behavior of rounddown_pow_of_two() for an input of 0 is undefined,
so handle that case and small values manually to achieve the same
results.
Signed-off-by: Christian Göttsche <cgzones@googlemail.com> Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com> Signed-off-by: Paul Moore <paul@paul-moore.com>