io_uring: inline io_req_work_grab_env()

The only caller of io_req_work_grab_env() is io_prep_async_work(), and
they are both initialising req->work. Inline grab_env(), it's easier
to keep this way, moreover there already were bugs with misplacing
io_req_init_async().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>