Update. Shortcut by using ARP in domain of FEC enabled.

BTW, shortcut is performed by use of ARP in two different
modes. Either by MPLS ARP, where its implementation remains in
netmpls/mpls_arp.c, or by ARP in domain of FEC, e.g. IPv4 based ARP.
	Any gateway address of given MPLS routes, except those denotes MPLS
ingress routes, is extended by gateway address in FEC by call of
mpls_rt_setfecgate() during runtime of mpls_rt_output(), iff call of
rtrequest1_fib() was successfull. this function wraps up rt_setgate()
and applies following algorithm:

  1. Backup rn_key of focussed rtentry{}, because during rt_setgate()
       memory chunk which stores key together with its gateway address
       are freed and reallocated.
  2. Determine length of gateway address (denoted by gwlen)
      and backup to rtentry{} mapped gateay address.
  3. Get length (denoted by gw0len) of gateway address
      in FEC (denoted by gateway0).
  4. Inkrement FTN by gwlen and catenete gateway address
      in FEC.
  5. Increase length of FTN by gw0len.
  6. Finally, call of rt_setgate() takes place.
  7. If called operation was successfull, restore original
      size of FTN and apply address of rtentry{} to nrt.

  + rn_key(rt)       + rt->rt_gateway
  |                  |                  + rt0->rt_gateway
  |                  |                  |
  v                  v                  v
  +------------------+------------------+---------------------+
  | LSP in           | LSP out, swap    | NH in domain of FEC |
  +------------------+------------------+---------------------+

Switching between use of different ARP is handled by sysctl(8):

	# sysctl net.link.ether.mpls.use_arp_in_fec=1

enables use of ARP relating domain of FEC. Unsetting disables and
causes fallback by using MPLS ARP, which is default.