rtnetlink: Add bulk registration helpers for rtnetlink message handlers.

[ Upstream commit 07cc7b0b ]

Before commit addf9b90 ("net: rtnetlink: use rcu to free rtnl message
handlers"), once rtnl_msg_handlers[protocol] was allocated, the following
rtnl_register_module() for the same protocol never failed.

However, after the commit, rtnl_msg_handler[protocol][msgtype] needs to
be allocated in each rtnl_register_module(), so each call could fail.

Many callers of rtnl_register_module() do not handle the returned error,
and we need to add many error handlings.

To handle that easily, let's add wrapper functions for bulk registration
of rtnetlink message handlers.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: d5170561 ("mctp: Handle error of rtnl_register_module().")
Signed-off-by: Sasha Levin <sashal@kernel.org>