sched: Fix herding issue

check_for_migration() could run concurrently on multiple cpus,
resulting in multiple tasks wanting to migrate to same cpu. This could
cause cpus to be underutilized and lead to increased scheduling
latencies for tasks. Fix this by serializing select_best_cpu() calls
from cpus running check_for_migration() check and marking selected
cpus as reserved, so that subsequent call to select_best_cpu() from
check_for_migration() will skip reserved cpus.

Change-Id: I73a22cacab32dee3c14267a98b700f572aa3900c
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>