rtnetlink: Call validate_linkmsg() in do_setlink().

There are 3 paths that finally call do_setlink(), and validate_linkmsg()
is called in each path.

  1. RTM_NEWLINK
    1-1. dev is found in __rtnl_newlink()
    1-2. dev isn't found, but IFLA_GROUP is specified in
          rtnl_group_changelink()
  2. RTM_SETLINK

The next patch factorises 1-1 to a separate function.

As a preparation, let's move validate_linkmsg() calls to do_setlink().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>