io_uring: only test SQPOLL cpu after we've verified it

We currently call cpu_possible() even if we don't use the CPU. Move the
test under the SQ_AFF branch, which is the only place where we'll use
the value. Do the cpu_possible() test AFTER we've limited it to a max
of NR_CPUS. This avoids triggering the following warning:

WARNING: CPU: 1 PID: 7600 at include/linux/cpumask.h:121 cpu_max_bits_warn

if CONFIG_DEBUG_PER_CPU_MAPS is enabled.

While in there, also move the SQ thread idle period assignment inside
SETUP_SQPOLL, as we don't use it otherwise either.

Reported-by:  <syzbot+cd714a07c6de2bc34293@syzkaller.appspotmail.com>
Fixes: 6c271ce2

 ("io_uring: add submission polling")
Signed-off-by: Jens Axboe <axboe@kernel.dk>