linux-cp: Fix add vs update on routes

linux-cp: Fix add vs update on routes

Linux uses NLM_F_REPLACE in the netlink message to signal a FIB update
The code invariably does a FIB update for IPv4 and a addition for IPv6.
Without this fix, the following:
 ip route add 2001:db8::/48 via 2001:db8::1
 ip route replace 2001:db8::/48 via 2001:db8::2

ends up as two separate FIB entries in VPP. With the fix, there will be one FIB entry (the second one with nexthop ::2).

Type: fix
Change-Id: I8f98d6ded52ae0c60bfddaa7fc39acbbaa19d34a
Signed-off-by: Pim van Pelt <pim@ipng.nl>
(cherry picked from commit af4fa965e909db69ecde9deb6b7f9a53553aeccb)