BUG/MAJOR: pools: fix incomplete backport of lockless pool fix

BUG/MAJOR: pools: fix incomplete backport of lockless pool fix

Commit bc76411e0 ("BUG/MAJOR: pools: fix possible race with free() in
the lockless variant") wasn't complete. The __pool_free() function also
needed a part of the lockless variant of the code that was in
pool_put_to_shared_cache() in 2.4. Without it, a __pool_free() during
a contented pool_alloc() may catch free_list == POOL_BUSY and copy it
into the updated free list, so that the next call to __pool_get_first()
loops forever and gets killed by the watchdog.

Sadly it requires load under contention on a non-glibc system to notice
it :-/

This fix is only for 2.3 and 2.2 since 2.2 now also contains the faulty
backport of the patch above.