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

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.

Reviewed by: chs
Differential Revision: https://reviews.freebsd.org/D23182