io_uring/kbuf: recycle freed mapped buffer ring entries

Right now we stash any potentially mmap'ed provided ring buffer range
for freeing at release time, regardless of when they get unregistered.
Since we're keeping track of these ranges anyway, keep track of their
registration state as well, and use that to recycle ranges when
appropriate rather than always allocate new ones.

The lookup is a basic scan of entries, checking for the best matching
free entry.

Fixes: c392cbec

 ("io_uring/kbuf: defer release of mapped buffer rings")
Signed-off-by: Jens Axboe <axboe@kernel.dk>