MINOR: pools: always use atomic ops to maintain counters

MINOR: pools: always use atomic ops to maintain counters

A part of the code cannot be factored out because it still uses non-atomic
inc/dec for pool->used and pool->allocated as these are located under the
pool's lock. While it can make sense in terms of bus cycles, it does not
make sense in terms of code normalization. Further, some operations were
still performed under a lock that could be totally removed via the use of
atomic ops.

There is still one occurrence in pool_put_to_shared_cache() in the locked
code where pool_free_area() is called under the lock, which must absolutely
be fixed.