MINOR: memory: move pool-specific path of the locked pool_free() to __pool_free()

MINOR: memory: move pool-specific path of the locked pool_free() to __pool_free()

pool_free() was not identical between locked and lockless pools. The
different was the call to __pool_free() in one case versus open-coded
accesses in the other, and the poisoning brought by commit da52035a45
("MINOR: memory: also poison the area on freeing") which unfortunately
did if only for the lockless path.

Let's now have __pool_free() to work on the global pool also in the
locked case so that the code is architected similarly.