io_uring/poll: unify poll waitqueue entry and list removal

io_uring/poll: unify poll waitqueue entry and list removal

For some cases, the order in which the waitq entry list and head
writing happens is important, for others it doesn't really matter.
But it's somewhat confusing to have them spread out over the file.

Abstract out the nicely documented code in io_pollfree_wake() and
move it into a helper, and use that helper consistently rather than
having other call sites manually do the same thing. While at it,
correct a comment function name as well.

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