net: geneve: modify IP header check in geneve6_xmit_skb and geneve_xmit_skb

Modify the header size check in geneve6_xmit_skb and geneve_xmit_skb
to use pskb_inet_may_pull rather than pskb_network_may_pull. This fixes
two kernel selftest failures introduced by the commit introducing the
checks:
IPv4 over geneve6: PMTU exceptions
IPv4 over geneve6: PMTU exceptions - nexthop objects

It does this by correctly accounting for the fact that IPv4 packets may
transit over geneve IPv6 tunnels (and vice versa), and still fixes the
uninit-value bug fixed by the original commit.

Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: 6628ddfe ("net: geneve: check skb is large enough for IPv4/IPv6 header")
Suggested-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>