src: add fib expression

src: add fib expression

This adds the 'fib' expression which can be used to
obtain the output interface from the route table based on either
source or destination address of a packet.

This can be used to e.g. add reverse path filtering:

 # drop if not coming from the same interface packet
 # arrived on
 # nft add rule x prerouting fib saddr . iif oif eq 0 drop

 # accept only if from eth0
 # nft add rule x prerouting fib saddr . iif oif eq "eth0" accept

 # accept if from any valid interface
 # nft add rule x prerouting fib saddr oif accept

Querying of address type is also supported.  This can be used
to e.g. only accept packets to addresses configured in the same
interface:
 # fib daddr . iif type local

Its also possible to use mark and verdict map, e.g.:
 # nft add rule x prerouting meta mark set 0xdead fib daddr . mark type vmap {
   blackhole : drop,
   prohibit : drop,
   unicast : accept
 }

Signed-off-by: Florian Westphal <fw@strlen.de>