net: rtnetlink: use netif_oper_up instead of open code

The open code is defined as a new helper function(netif_oper_up) on netdev.h,
the code is dev->operstate == IF_OPER_UP || dev->operstate == IF_OPER_UNKNOWN.
Thus, replace the open code to netif_oper_up. This patch doesn't change logic.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Link: https://lore.kernel.org/r/20220831125845.1333-1-claudiajkang@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>