geneve: Don't assume linear buffers in error handler

geneve: Don't assume linear buffers in error handler

In commit 88b9ae65ba5a ("geneve: ICMP error lookup handler") I wrongly
assumed buffers from icmp_socket_deliver() would be linear. This is not
the case: icmp_socket_deliver() only guarantees we have 8 bytes of linear
data.

Eric fixed this same issue for fou and fou6 in commits c626a4560f43
("fou, fou6: do not assume linear skbs") and eb1b70caa60e ("fou, fou6:
avoid uninit-value in gue_err() and gue6_err()").

Use pskb_may_pull() instead of checking skb->len, and take into account
the fact we later access the GENEVE header with udp_hdr(), so we also
need to sum skb_transport_header() here.

Reported-by: Guillaume Nault <gnault@redhat.com>
Fixes: 88b9ae65ba5a ("geneve: ICMP error lookup handler")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>