MINOR: pools: factor the release code into pool_put_to_os()

MINOR: pools: factor the release code into pool_put_to_os()

There are two levels of freeing to the OS:
  - code that wants to keep the pool's usage counters updated uses
    pool_free_area() and handles the counters itself. That's what
    pool_put_to_shared_cache() does in the no-global-pools case.
  - code that does not want to update the counters because they were
    already updated only calls pool_free_area().

Let's extract these calls to establish the symmetry with pool_get_from_os()
and pool_alloc_nocache(), resulting in pool_put_to_os() (which only updates
the allocated counter) and pool_free_nocache() (which also updates the used
counter). This will later allow to simplify the generic code.