MINOR: task: split the counts of local and global tasks picked

MINOR: task: split the counts of local and global tasks picked

In process_runnable_tasks() we're still calling __task_unlink_rq() to
pick a task, and this function tries to guess where to pick the task
from and which counter to update while the caller's context already
has everything. Worse, the number of local tasks is decremented then
recredited, doubling the operations. In order to avoid this we first
need to keep separate counters for local and global tasks that were
picked. This is what this patch does.