io_uring/zcrx: return ifq id to the user

io_uring/zcrx: return ifq id to the user

IORING_OP_RECV_ZC requests take a zcrx object id via sqe::zcrx_ifq_idx,
which binds it to the corresponding if / queue. However, we don't return
that id back to the user. It's fine as currently there can be only one
zcrx and the user assumes that its id should be 0, but as we'll need
multiple zcrx objects in the future let's explicitly pass it back on
registration.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8714667d370651962f7d1a169032e5f02682a73e.1744722517.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>