io_uring/poll: pull ownership handling into poll.h

io_uring/poll: pull ownership handling into poll.h

In preparation for using it from somewhere else. Rather than try and
duplicate the functionality, just make it generically available to
io_uring opcodes.

Note: would have to be used carefully, cannot be used by opcodes that
can trigger poll logic.

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