gre: set proper fib index for unnumbered interfaces, unset fib index before forwardin...

gre: set proper fib index for unnumbered interfaces, unset fib index before forwarding gre payload

This commit introduces 2 fixes:
1) After GRE decapsulation sw_if_index[VLIB_TX] is set as fib index of GRE tunnel.
But since GRE tunnel can work on v4 endpoints and have v6 payload, we need to reset it.
In case we get IPv6 packet inside IPv4 GRE tunnel (or vice-versa) fib index can be (and usually is) invalid.
2) Check that ip-table and ip6-table are the same when setting interface as an unnumbered one.

Also, fix for the pipe test include setting the right unnumbered interface for the pipes

Type: fix

Signed-off-by: Stanislav Zaikin <zstaseg@gmail.com>
Change-Id: Id13d239cfdd21e0db6b1c9725f01c40d4af4d800