io_uring: kill REQ_F_LTIMEOUT_ACTIVE

Instead of handling double consecutive linked timeouts through tricky
flag combinations, just check the submit_state.link during timeout_prep
and fail that case in advance.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/04150760b0dc739522264b8abd309409f7421a06.1628981736.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>