sched/fair: fix "BUG: sleeping function called from invalid context"

Commit 39d60ed9 ("sched/fair: Add snapshot of placement changes")
introduced a bug where we end up calling rcu_read_unlock() without
calling rcu_read_lock() and leading to warning "BUG: sleeping function
called from invalid context" while trying to find the energy efficient CPU
for the tasks.

Change-Id: I727d11660ed43341751dfb6e6f8d23968dfe9b49
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>