mlxsw: spectrum_router: Remove unlikely user-triggerable warning

In case the driver vetoes the addition of an IPv6 multipath route, the
IPv6 stack will emit delete notifications for the sibling routes that
were already added to the FIB trie. Since these siblings are not present
in hardware, a warning will be generated.

Have the driver ignore notifications for routes it does not have.

Fixes: ebee3cad

 ("ipv6: Add IPv6 multipath notifications for add / replace")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>