io_uring/mock: add trivial poll handler

Add a flag that enables polling on the mock file. For now it's trivially
says that there is always data available, it'll be extended in the
future.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f16de043ec4876d65fae294fc99ade57415fba0c.1750599274.git.asml.silence@gmail.com


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