MINOR: task: make grq_total atomic to move it outside of the grq_lock

MINOR: task: make grq_total atomic to move it outside of the grq_lock

Instead of decrementing grq_total once per task picked from the global
run queue, let's do it at once after the loop like we do for other
counters. This simplifies the code everywhere. It is not expected to
bring noticeable improvements however, since global tasks tend to be
less common nowadays.