BUG/MEDIUM: tasks: Make sure we don't schedule a task already running

BUG/MEDIUM: tasks: Make sure we don't schedule a task already running

In task_schedule(), before attempting to set the new task expiration
date, make sure it is not running by trying to set the TASK_RUNNING
flag, and waiting if it is already there. Having the flag set will
ensure that the task won't be running while we're modifying it.
There is a very rare race condition, where the expire would be set by
task_schedule(), then the running task might set it to something else,
and if it sets it to TICK_ETERNITY before task_schedule() calls
__task_queue(), then we will hit a BUG_ON() there.
This is very hard to reproduce, but has been reported a few times,
included in Github issue #3327, which should now be fixed.

This should be backported as far back as 2.8.

WIP: Make sure the task is not running before changing expire