task_work: Replace spin_unlock_wait() with lock/unlock pair

There is no agreed-upon definition of spin_unlock_wait()'s semantics,
and it appears that all callers could do just as well with a lock/unlock
pair.  This commit therefore replaces the spin_unlock_wait() call in
task_work_run() with a spin_lock_irq() and a spin_unlock_irq() aruond
the cmpxchg() dequeue loop.  This should be safe from a performance
perspective because ->pi_lock is local to the task and because calls to
the other side of the race, task_work_cancel(), should be rare.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>