io_uring: remove unnecessary variable

'running' is set once and read once, so can easily just remove it

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220830125013.570060-2-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>