net: rtnetlink - make create_link take name_assign_type

This passes down NET_NAME_USER (or NET_NAME_ENUM) to alloc_netdev(),
for any device created over rtnetlink.

v9: restore reverse-christmas-tree order of local variables

Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David S. Miller <davem@davemloft.net>