io_uring/zcrx: add copy fallback

There are scenarios in which the zerocopy path can get a kernel buffer
instead of a net_iov and needs to copy it to the user, whether it is
because of mis-steering or simply getting an skb with the linear part.
In this case, grab a net_iov, copy into it and return it to the user as
normally.

At the moment the user doesn't get any indication whether there was a
copy or not, which is left for follow up work.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David Wei <dw@davidwei.uk>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20250215000947.789731-10-dw@davidwei.uk


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