sched: Fix "releasing a pinned lock" lockdep warning

WALT requires both src and dst runqueue locks to be held
during migration. So a double_lock_balance() is added
in move_queued_task() before calling set_task_cpu(). However,
releasing the src rq lock, which is pinned is giving a
lockdep warning. Fix this by unpinning the lock before
calling double_lock_balance.

Change-Id: I64dec0701b3467185cf53f311ebb521c6a822e88
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>