io_uring: make req->timeout be dynamically allocated

There are a few reasons for this:

- As a prep to improving the linked timeout logic
- io_timeout is the biggest member in the io_kiocb opcode union

This also enables a few cleanups, like unifying the timer setup between
IORING_OP_TIMEOUT and IORING_OP_LINK_TIMEOUT, and not needing multiple
arguments to the link/prep helpers.

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