This function is very common in a debug build. I can even see it in
profiler.

This patch reduces execution time of fil_validate() from
8948ns
8367ns
8650ns
8906ns
8448ns

to
260ns
232ns
403ns
275ns
169ns
in my environment.

The trick is a faster fil_space_t iteration. Hash table
is typically initialized with a size of 50,000. And looping through
it is slow. Slower, than iterating an exact amount of fil_space_t
which is typically less than ten.

Only debug builds are affected.