sctp: increase sk_wmem_alloc when head->truesize is increased

I changed to count sk_wmem_alloc by skb truesize instead of 1 to
fix the sk_wmem_alloc leak caused by later truesize's change in
xfrm in Commit 02968ccf ("sctp: count sk_wmem_alloc by skb
truesize in sctp_packet_transmit").

But I should have also increased sk_wmem_alloc when head->truesize
is increased in sctp_packet_gso_append() as xfrm does. Otherwise,
sctp gso packet will cause sk_wmem_alloc underflow.

Fixes: 02968ccf ("sctp: count sk_wmem_alloc by skb truesize in sctp_packet_transmit")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>