MINOR: pools: move the failed allocation counter over a few buckets

MINOR: pools: move the failed allocation counter over a few buckets

The failed allocation counter cannot depend on a pointer, but since it's
a perpetually increasing counter and not a gauge, we don't care where
it's incremented. Thus instead we're hashing on the TID. There's no
contention there anyway, but it's better not to waste the room in
the pool's heads and to move that with the other counters.