net: rtnetlink: add helper to extract msg type's kind

[ Upstream commit 2e9ea3e3 ]

Add a helper which extracts the msg type's kind using the kind mask (0x3).

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>