net: loopback: Hold rtnl_net_lock() in blackhole_netdev_init().

blackhole_netdev is the global device in init_net.

Let's hold rtnl_net_lock(&init_net) in blackhole_netdev_init().

While at it, the unnecessary dev_net_set() call is removed, which
is done in alloc_netdev_mqs().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250114081352.47404-1-kuniyu@amazon.com


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