sched: Make migration_call() safe for stop_machine()-free hotplug

sched: Make migration_call() safe for stop_machine()-free hotplug

The CPU_DYING branch of migration_call() relies on the fact that
CPU-hotplug offline operations use stop_machine().  This commit therefore
attempts to remedy this situation by acquiring the relevant runqueue
locks.  Note that sched_ttwu_pending() remains outside of the scope of
these new runqueue-lock critical sections because (1) sched_ttwu_pending()
does its own runqueue-lock acquisition and (2) sched_ttwu_pending() handles
pending wakeups, and no further wakeups can select this CPU because it
is already marked as offline.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>