bitops: Use non-bitops ctzl

The use of ctz has already eliminated zero, and thus the difference
in edge conditions between the two routines is irrelevant.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>