MINOR: pools: move pool_free_area() out of the lock in the locked version

MINOR: pools: move pool_free_area() out of the lock in the locked version

Calling pool_free_area() inside a lock in pool_put_to_shared_cache() is
a very bad idea. Fortunately this only happens on the lowest end platforms
which almost never use threads or in very small counts.

This change consists in zeroing the pointer once already released to the
cache in the first test so that the second stage knows if it needs to
pass it to the OS or not. This has slightly reduced the length of the