geneve: make sure to pull inner header in geneve_rx()

In the Linux kernel, the following vulnerability has been resolved:

geneve: make sure to pull inner header in geneve_rx()

syzbot triggered a bug in geneve_rx() [1]

Issue is similar to the one I fixed in commit 8d975c15c0cd
("ip6_tunnel: make sure to pull inner header in __ip6_tnl_rcv()")

We have to save skb->network_header in a temporary variable
in order to be able to recompute the network_header pointer
after a pskb_inet_may_pull() call.

pskb_inet_may_pull() makes sure the needed headers are in skb->head.

[1]
BUG: KMSAN: uninit-value in IP_ECN_decapsulate include/net/inet_ecn.h:302 [inline]
BUG: KMSAN: uninit-value in geneve_rx drivers/net/geneve.c:279 [inline]
BUG: KMSAN: uninit-value in geneve_udp_encap_recv+0x36f9/0x3c10 drivers/net/geneve.c:391
IP_ECN_decapsulate include/net/inet_ecn.h:302 [inline]
geneve_rx drivers/net/geneve.c:279 [inline]
geneve_udp_encap_recv+0x36f9/0x3c10 drivers/net/geneve.c:391
udp_queue_rcv_one_skb+0x1d39/0x1f20 net/ipv4/udp.c:2108
udp_queue_rcv_skb+0x6ae/0x6e0 net/ipv4/udp.c:2186
udp_unicast_rcv_skb+0x184/0x4b0 net/ipv4/udp.c:2346
__udp4_lib_rcv+0x1c6b/0x3010 net/ipv4/udp.c:2422
udp_rcv+0x7d/0xa0 net/ipv4/udp.c:2604
ip_protocol_deliver_rcu+0x264/0x1300 net/ipv4/ip_input.c:205
ip_local_deliver_finish+0x2b8/0x440 net/ipv4/ip_input.c:233
NF_HOOK include/linux/netfilter.h:314 [inline]
ip_local_deliver+0x21f/0x490 net/ipv4/ip_input.c:254
dst_input include/net/dst.h:461 [inline]
ip_rcv_finish net/ipv4/ip_input.c:449 [inline]
NF_HOOK include/linux/netfilter.h:314 [inline]
ip_rcv+0x46f/0x760 net/ipv4/ip_input.c:569
__netif_receive_skb_one_core net/core/dev.c:5534 [inline]
__netif_receive_skb+0x1a6/0x5a0 net/core/dev.c:5648
process_backlog+0x480/0x8b0 net/core/dev.c:5976
__napi_poll+0xe3/0x980 net/core/dev.c:6576
napi_poll net/core/dev.c:6645 [inline]
net_rx_action+0x8b8/0x1870 net/core/dev.c:6778
__do_softirq+0x1b7/0x7c5 kernel/softirq.c:553
do_softirq+0x9a/0xf0 kernel/softirq.c:454
__local_bh_enable_ip+0x9b/0xa0 kernel/softirq.c:381
local_bh_enable include/linux/bottom_half.h:33 [inline]
rcu_read_unlock_bh include/linux/rcupdate.h:820 [inline]
__dev_queue_xmit+0x2768/0x51c0 net/core/dev.c:4378
dev_queue_xmit include/linux/netdevice.h:3171 [inline]
packet_xmit+0x9c/0x6b0 net/packet/af_packet.c:276
packet_snd net/packet/af_packet.c:3081 [inline]
packet_sendmsg+0x8aef/0x9f10 net/packet/af_packet.c:3113
sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg net/socket.c:745 [inline]
__sys_sendto+0x735/0xa10 net/socket.c:2191
__do_sys_sendto net/socket.c:2203 [inline]
__se_sys_sendto net/socket.c:2199 [inline]
__x64_sys_sendto+0x125/0x1c0 net/socket.c:2199
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was created at:
slab_post_alloc_hook mm/slub.c:3819 [inline]
slab_alloc_node mm/slub.c:3860 [inline]
kmem_cache_alloc_node+0x5cb/0xbc0 mm/slub.c:3903
kmalloc_reserve+0x13d/0x4a0 net/core/skbuff.c:560
__alloc_skb+0x352/0x790 net/core/skbuff.c:651
alloc_skb include/linux/skbuff.h:1296 [inline]
alloc_skb_with_frags+0xc8/0xbd0 net/core/skbuff.c:6394
sock_alloc_send_pskb+0xa80/0xbf0 net/core/sock.c:2783
packet_alloc_skb net/packet/af_packet.c:2930 [inline]
packet_snd net/packet/af_packet.c:3024 [inline]
packet_sendmsg+0x70c2/0x9f10 net/packet/af_packet.c:3113
sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg net/socket.c:745 [inline]
__sys_sendto+0x735/0xa10 net/socket.c:2191
__do_sys_sendto net/socket.c:2203 [inline]
__se_sys_sendto net/socket.c:2199 [inline]
__x64_sys_sendto+0x125/0x1c0 net/socket.c:2199
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x63/0x6b