io_uring: don't punt files update to io-wq unconditionally

There's no reason to punt it unconditionally, we just need to ensure that
the submit lock grabbing is conditional.

Fixes: 05f3fb3c ("io_uring: avoid ring quiesce for fixed file set unregister and update")
Signed-off-by: Jens Axboe <axboe@kernel.dk>