BACKPORT: 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>
(cherry picked from commit b08a784a)
Bug: 421244320
[willdeacon@: Fix context conflict with __zerocopy_sg_from_iter()]
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I1a19947a273bf88f57474974d8cd752dc13246ee