libceph: move more code into __complete_request()

Move req->r_completion wake up and req->r_kref decrement into
__complete_request().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>