Users may specify a CPU where the sqpoll thread would run. This may
conflict with cpuset operations because of strict PF_NO_SETAFFINITY
requirement. That flag is unnecessary for polling "kernel" threads, see
the reasoning in commit 01e68ce08a30 ("io_uring/io-wq: stop setting
PF_NO_SETAFFINITY on io-wq workers"). Drop the flag on poll threads too.