net/ipv6: Revert attempt to simplify route replace and append

NetworkManager likes to manage linklocal prefix routes and does so with
the NLM_F_APPEND flag, breaking attempts to simplify the IPv6 route
code and by extension enable multipath routes with device only nexthops.

Revert f34436a4 and these followup patches:
6eba08c3 ("ipv6: Only emit append events for appended routes").
ce45bded ("mlxsw: spectrum_router: Align with new route replace logic")
53b562df ("mlxsw: spectrum_router: Allow appending to dev-only routes")

Update the fib_tests cases to reflect the old behavior.

Fixes: f34436a4 ("net/ipv6: Simplify route replace and appending into multipath route")
Signed-off-by: David Ahern <dsahern@gmail.com>