vm_phys: use ilog2(x) instead of fls(x)-1

[dougm]

Summary

One of these changes saves two instructions on an amd64 GENERIC-NODEBUG build. The rest are entirely cosmetic, because the compiler can deduce that x is nonzero, and avoid the needless test.

[alc]

Please coordinate this with @markj given his lazy init change.

[markj]

In D45331#1037497, @alc wrote:

Please coordinate this with @markj given his lazy init change.

Please go ahead. It'll be easy to rebase past this patch and I need to retest before pushing.