lib/find: optimize for_each() macros

Moving an iterator of the macros inside conditional part of for-loop
helps to generate a better code. It had been first implemented in commit
7baac8b9 ("cpumask: make for_each_cpu_mask a bit smaller").

Now that cpumask for-loops are the aliases to bitmap loops, it's worth
to optimize them the same way.

Bloat-o-meter says:
add/remove: 8/12 grow/shrink: 147/592 up/down: 4876/-24416 (-19540)

Signed-off-by: Yury Norov <yury.norov@gmail.com>