vsock/virtio: fix MSG_ZEROCOPY pinned-pages accounting

In the Linux kernel, the following vulnerability has been resolved:

vsock/virtio: fix MSG_ZEROCOPY pinned-pages accounting

virtio_transport_init_zcopy_skb() uses iter->count as the size argument

for msg_zerocopy_realloc(), which in turn passes it to

mm_account_pinned_pages() for RLIMIT_MEMLOCK accounting. However, this

function is called after virtio_transport_fill_skb() has already consumed

the iterator via __zerocopy_sg_from_iter(), so on the last skb, iter->count

will be 0, skipping the RLIMIT_MEMLOCK enforcement.

Pass pkt_len (the total bytes being sent) as an explicit parameter to

virtio_transport_init_zcopy_skb() instead of reading the already-consumed

iter->count.

This matches TCP and UDP, which both call msg_zerocopy_realloc() with

the original message size.