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

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

Commit bc76411e0 ("BUG/MAJOR: pools: fix possible race with free() in
the lockless variant") was missing another unprotected access to the
pool's free_list in __pool_refill_alloc() because this one was completely
dropped from 2.4 and above. There we need to loop over POOL_BUSY just like
in the __pool_free() code, otherwise we risk to insert such a POOL_BUSY
into the list.

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