powerof2: replace loops with fls or ilog2

In several places, a loop tests for powers of two, or iterates through
powers of two. In those places, replace the loop with an invocation
of fls or ilog2 without changing the meaning of the code.

Reviewed by: alc, markj, kib, np, erj, avg (previous version)
Differential Revision: https://reviews.freebsd.org/D45494