io_uring: preparation for rsrc tagging

We need a way to notify userspace when a lazily removed resource
actually died out. This will be done by associating a tag, which is u64
exactly like req->user_data, with each rsrc (e.g. buffer of file). A CQE
will be posted once a resource is actually put down.

Tag 0 is a special value set by default, for whcih it don't generate an
CQE, so providing the old behaviour.

Don't expose it to the userspace yet, but prepare internally, allocate
buffers, add all posting hooks, etc.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2e6beec5eabe7216bb61fb93cdf5aaf65812a9b0.1619356238.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>