We only want to send the speedup to the lower layers when there's a shortage.

[imp]

Summary

Only send a speedup when there's a shortage. While this is a little racy, lost
races aren't a big deal for this function. If there's a shorage just popping up
after we check these values, then we'll catch it next time. If there's a
shortage that's just clearing up, we may do some work at the lower layers a
little sooner than we otherwise would have. Sicne shortages are relatively rare
events, both races are acceptable.

Test Plan

Without these, I'm seeing way too many speedups.

I tested this with:
dtrace -n 'fbt::softdep_send_speedup:entry {stack()}'

before the change, I saw it called all the time. After, it wasn't called at all.

Thanks for chuck for noticing the myriad of issues here. Other reviews will fix the other ones.

[chs]

looks good.