Modify return value of snat_out2in_lb in order  to trace session_index

02:06:32:674656: ip4-input
  TCP: 5.1.1.1 -> 4.1.1.5
    tos 0x00, ttl 64, length 60, checksum 0x116f
    fragment id 0x1e46, flags DONT_FRAGMENT
02:06:32:674668: nat44-out2in
  NAT44_OUT2IN: sw_if_index 2, next index 1, session index -1   //it's inaccurate
02:06:32:674790: ip4-lookup
  fib 0 dpo-idx 0 flow hash: 0x00000000
  TCP: 5.1.1.1 -> 4.1.1.3
    tos 0x00, ttl 64, length 60, checksum 0x1171
    fragment id 0x1e46, flags DONT_FRAGMENT