netfilter: nf_tables: restore IP sanity checks for netdev/egress

netfilter: nf_tables: restore IP sanity checks for netdev/egress

Subtract network offset to skb->len before performing IPv4 header sanity
checks, then adjust transport offset from offset from mac header.

Jorge Ortiz says:

When small UDP packets (< 4 bytes payload) are sent from eth0,
`meta l4proto udp` condition is not met because `NFT_PKTINFO_L4PROTO` is
not set. This happens because there is a comparison that checks if the
transport header offset exceeds the total length.  This comparison does
not take into account the fact that the skb network offset might be
non-zero in egress mode (e.g., 14 bytes for Ethernet header).

Fixes: 0ae8e4cca787 ("netfilter: nf_tables: set transport offset from mac header for netdev/egress")
Reported-by: Jorge Ortiz <jorge.ortiz.escribano@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>