io_uring/rsrc: encode node type and ctx together

Rather than keep the type field separate rom ctx, use the fact that we
can encode up to 4 types of nodes in the LSB of the ctx pointer. Doesn't
reclaim any space right now on 64-bit archs, but it leaves a full int
for future use.

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