rtnl: split __rtnl_newlink() into two functions

__rtnl_newlink() is 250LoC, but has a few clear sections.
Move the part which creates a new netdev to a separate
function.

For ease of review code will be moved in the next change.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>