sched: walt: Serialize check_for_migration across all CPUs

check_for_migration may select the same destination cpu for two separate
tasks running on two separate cpus, resulting in those two tasks being
packed onto the same higher-capacity CPU. Avoid this situation by
serializing check_for_migration with a global spinlock.

Change-Id: I4ec439bcbcdc0a52a5289f239a6160baa7e50da0
Signed-off-by: Puja Gupta <pujag@codeaurora.org>