macvtap: zerocopy: fix offset calculation when building skb

commit 3afc9621f15701c557e60f61eba9242bac2771dd upstream.

This patch fixes the offset calculation when building skb:

- offset1 were used as skb data offset not vector offset
- reset offset to zero only when we advance to next vector

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>