CLEANUP: pools: re-merge pool_refill_alloc() and __pool_refill_alloc()

CLEANUP: pools: re-merge pool_refill_alloc() and __pool_refill_alloc()

They were strictly equivalent, let's remerge them and rename them to
pool_alloc_nocache() as it's the call which performs a real allocation
which does not check nor update the cache. The only difference in the
past was the former taking the lock and not the second but now the lock
is not needed anymore at this stage since the pool's list is not touched.

In addition, given that the "avail" argument is no longer used by the
function nor by its callers, let's drop it.