net: Introduce skb_copy_datagram_from_iter_full()

net: Introduce skb_copy_datagram_from_iter_full()

In a similar manner to copy_from_iter()/copy_from_iter_full(), introduce
skb_copy_datagram_from_iter_full() which reverts the iterator to its
initial state when returning an error.

A subsequent fix for a vsock regression will make use of this new
function.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Will Deacon <will@kernel.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://patch.msgid.link/20250818180355.29275-2-will@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>