Adds support for RTA_MARK rt attribute added in linux-2.6.36

$ ip route get ADDR mark 4
192.168.20.110 dev eth1  src 192.168.20.108  mark 4
    cache  mtu 1500 advmss 1460 hoplimit 64

$ ip route get 192.168.20.108 from ADDR iif STRING mark 256
local 192.168.20.108 from 192.168.20.110 dev lo  src 192.168.20.108  mark 0x100
    cache <local,src-direct>  iif eth1

$ ip route list cache [ADDR] mark NUMBER

Hexadecimal output if mark >= 16
null marks are not displayed.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>