net: bridge: fix multicast-to-unicast with fraglist GSO

Calling skb_copy on a SKB_GSO_FRAGLIST skb is not valid, since it returns
an invalid linearized skb. This code only needs to change the ethernet
header, so pskb_copy is the right function to call here.

Fixes: 6db6f0ea

 ("bridge: multicast to unicast")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>