sched: rt: Use RCU lock in rt task cpu select path

RT energy aware cpu selection tries to get cpu idle state
without holding rcu read lock, it may get stale/corrupted value.

Fix it by moving rcu unlock to end for energy aware cpu
selection.

Change-Id: Ia219062db6b97b9a44681647cea077c45c29cff2
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>