blkcg: don't hold blkcg lock when deactivating policy

blkcg: don't hold blkcg lock when deactivating policy

As described in the comment of blkcg_activate_policy(),
*Update of each blkg is protected by both queue and blkcg locks so
that holding either lock and testing blkcg_policy_enabled() is
always enough for dereferencing policy data.*
with queue lock held, there is no need to hold blkcg lock in
blkcg_deactivate_policy(). Similar case is in
blkcg_activate_policy(), which has removed holding of blkcg lock in
commit 4c55f4f9ad3001ac1fefdd8d8ca7641d18558e23.

Signed-off-by: Jiang Biao <[email protected]>
Signed-off-by: Wen Yang <[email protected]>
CC: Tejun Heo <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>