arp: Convert ioctl(SIOCGARP) to RCU.

ioctl(SIOCGARP) holds rtnl_lock() to get netdev by __dev_get_by_name()
and copy dev->name safely and calls neigh_lookup() later, which looks
up a neighbour entry under RCU.

Let's replace __dev_get_by_name() with dev_get_by_name_rcu() and strscpy()
with netdev_copy_name() to avoid locking rtnl_lock().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240430015813.71143-8-kuniyu@amazon.com


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