io_uring: don't vmalloc rsrc tags

We don't really need vmalloc for keeping tags, it's not a hot path and
is there out of convenience, so replace it with two level tables to not
litter kernel virtual memory mappings.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/241a3422747113a8909e7e1030eb585d4a349e0d.1623634181.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>