io_uring/rsrc: add last-lookup cache hit to io_rsrc_node_lookup()

io_uring/rsrc: add last-lookup cache hit to io_rsrc_node_lookup()

This avoids array_index_nospec() for repeated lookups on the same node,
which can be quite common (and costly). When the node sees its last
put, it'll clear the cache as well.

Note: need to double check this is 100% safe wrt speculation, but I
believe it should be as we're not using the passed in value to index
any arrays (directly).

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