netfilter: flowtable: validate vlan header

[ Upstream commit 6ea14ccb60c8ab829349979b22b58a941ec4a3ee ]

Ensure there is sufficient room to access the protocol field of the
VLAN header, validate it once before the flowtable lookup.

=====================================================
BUG: KMSAN: uninit-value in nf_flow_offload_inet_hook+0x45a/0x5f0 net/netfilter/nf_flow_table_inet.c:32
 nf_flow_offload_inet_hook+0x45a/0x5f0 net/netfilter/nf_flow_table_inet.c:32
 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
 nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626
 nf_hook_ingress include/linux/netfilter_netdev.h:34 [inline]
 nf_ingress net/core/dev.c:5440 [inline]

Fixes: 4cd91f7c ("netfilter: flowtable: add vlan support")
Reported-by:  <syzbot+8407d9bb88cd4c6bf61a@syzkaller.appspotmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>