GRE tunnel key should use fib_index instead of fib_id (VPP-1118)

The code path in src/vnet/gre/interface.c uses fib_id as a
component to generate hash key for GRE tunnel. This should be
fib_index as the GRE rx/decap data path will be using fib_index
to generate the hash key to lookup the GRE tunnel.

Change-Id: Ia7f0892d84f9dac79223a6e016775892b61eb5fb
Signed-off-by: John Lo <loj@cisco.com>
(cherry picked from commit 72247c803250894834d15952a6fddcd8f4a39a9c)