net: rtnetlink: add msg kind names

[ Upstream commit 12dc5c2c ]

Add rtnl kind names instead of using raw values. We'll need to
check for DEL kind later to validate bulk flag support.

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: bf29555f ("rtnetlink: Allow deleting FDB entries in user namespace")
Signed-off-by: Sasha Levin <sashal@kernel.org>