net: core: reject skb_copy(_expand) for fraglist GSO skbs
SKB_GSO_FRAGLIST skbs must not be linearized, otherwise they become
invalid. Return NULL if such an skb is passed to skb_copy or
skb_copy_expand, in order to prevent a crash on a potential later
call to skb_gso_segment.
Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.") Signed-off-by: Felix Fietkau <nbd@nbd.name> Signed-off-by: David S. Miller <davem@davemloft.net>