More LISP SD FIB and forwarding fixes

o) Avoid using explicit_fib_index in the dst (main) FIBs. It's used in
the IP6 lookup
o) use if_address_index instead of rewrite_header.node_index to store
tunnel indexes. This ensures the tunnel index is used in the signature
computation and thus avoids all complications/hacks needed to make src
adjacencies unique.
o) Fixed negative fwd entry route insertion

Change-Id: Ie56356f165b96dfa929da5672a3a429996366460
Signed-off-by: Florin Coras <fcoras@cisco.com>