arp: Validate netmask earlier for SIOCDARP and SIOCSARP in arp_ioctl().

When ioctl(SIOCDARP/SIOCSARP) is issued with ATF_PUBL, r.arp_netmask
must be 0.0.0.0 or 255.255.255.255.

Currently, the netmask is validated in arp_req_delete_public() or
arp_req_set_public() under rtnl_lock().

We have ATF_NETMASK test in arp_ioctl() before holding rtnl_lock(),
so let's move the netmask validation there.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240430015813.71143-3-kuniyu@amazon.com


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