io_uring/cmd: allow multishot polled commands

io_uring/cmd: allow multishot polled commands

Some commands like timestamping in the next patch can make use of
multishot polling, i.e. REQ_F_APOLL_MULTISHOT. Add support for that,
which is condensed in a single helper called io_cmd_poll_multishot().

The user who wants to continue with a request in a multishot mode must
call the function, and only if it returns 0 the user is free to proceed.
Apart from normal terminal errors, it can also end up with -EIOCBQUEUED,
in which case the user must forward it to the core io_uring. It's
forbidden to use task work while the request is executing in a multishot
mode.

The API is not foolproof, hence it's not exported to modules nor exposed
in public headers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/bcf97c31659662c72b69fc8fcdf2a88cfc16e430.1750065793.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>