ceph: fix kick_requests()

commit 282c105225ec3229f344c5fced795b9e1e634440 upstream

__do_request() may unregister the request. So we should update
iterator 'p' before calling __do_request()

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
[ Yan, Zheng: backport to 3.16 ]