tee: optee: avoid possible double list_del()

This bug occurs when:

- a new request arrives, one thread(let's call it A) is pending in
  optee_supp_req() with req->busy is initial value false.

- tee-supplicant is killed, then optee_supp_release() is called, this
  function calls list_del(&req->link), and set supp->ctx to NULL. And
  it also wake up process A.

- process A continues, it firstly checks supp->ctx which is NULL,
  then checks req->busy which is false, at last run list_del(&req->link).
  This triggers double list_del() and results kernel panic.

For solve this problem, we rename req->busy to req->in_queue, and
associate it with state of whether req is linked to supp->reqs. So we
can just only check req->in_queue to make decision calling list_del()
or not.

Signed-off-by: Zhizhou Zhang <zhizhouzhang@asrmicro.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>