ipv4: fix a potential use after free in gre_offload.c

ipv4: fix a potential use after free in gre_offload.c

pskb_may_pull() may change skb->data and make greh pointer oboslete;
so need to reassign greh;
but since first calling pskb_may_pull already ensured that skb->data
has enough space for greh, so move the reference of greh before second
calling pskb_may_pull(), to avoid reassign greh.

Fixes: 7a7ffbabf9("ipv4: fix tunneled VM traffic over hw VXLAN/GRE GSO NIC")
Cc: Wei-Chun Chao <[email protected]>
Signed-off-by: Li RongQing <[email protected]>
Acked-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>