common/mempool: tolerate sum < 0

common/mempool: tolerate sum < 0

If we are adding up the shards, and race with some allocations and
deallocations, we might get a negative sum.  E.g., with 2 shards, intially
[0,0],

- A: read shard 0 (0)
- B: allocate object on shard 0 (now 1)
- C: dallocate object on shard 1 (now -1)
- A: read shard 1 (-1)
- A: sum is -1

Signed-off-by: Sage Weil <sage@redhat.com>