io_uring: enable optimized link handling for IORING_OP_POLL_ADD

As introduced by commit:

ba816ad6 ("io_uring: run dependent links inline if possible")

enable inline dependent link running for poll commands.
io_poll_complete_work() is the most important change, as it allows a
linked sequence of { POLL, READ } (for example) to proceed inline
instead of needing to get punted to another async context. The
submission side only potentially matters for sqthread, but may as well
include that bit.

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