io_uring: refactor io_sq_thread()

Move needs_sched declaration into the block where it's used, so it's
harder to misuse/wrongfully reuse.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e4a07db1353ee38b924dd1b45394cf8e746130b4.1624543113.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>