io_uring: clean up io_kill_linked_timeout() locking

Avoid jumping through hoops to silence unused variable warnings, and
also fix sparse rightfully complaining about the locking context:

fs/io_uring.c:1593:39: warning: context imbalance in 'io_kill_linked_timeout' - unexpected unlock

Provide the functional helper as __io_kill_linked_timeout(), and have
separate the locking from it.

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