io_uring: move to using private ring references

io_uring: move to using private ring references

io_uring currently uses percpu refcounts for the ring reference. This
works fine, but exiting a ring requires an RCU grace period to lapse
and this slows down ring exit quite a lot.

Add a basic per-cpu counter for our references instead, and use that.
This is in preparation for doing a sync wait on on any request (notably
file) references on ring exit. As we're going to be waiting on ctx refs
going away as well with that, the RCU grace period wait becomes a
noticeable slowdown.

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