io_uring/epoll: add multishot support for IORING_OP_EPOLL_WAIT

io_uring/epoll: add multishot support for IORING_OP_EPOLL_WAIT

As with other multishot requests, submitting a multishot epoll wait
request will keep it re-armed post the initial trigger. This allows
multiple epoll wait completions per request submitted, every time
events are available. If more completions are expected for this
epoll wait request, then IORING_CQE_F_MORE will be set in the posted
cqe->flags.

For multishot, the request remains on the epoll callback waitqueue
head. This means that epoll doesn't need to juggle the ep->lock
writelock (and disable/enable IRQs) for each invocation of the
reaping loop. That should translate into nice efficiency gains.

Use by setting IORING_EPOLL_WAIT_MULTISHOT in the sqe->epoll_flags
member. Must be used with provided buffers.

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