net: evaluate net.ipv4.conf.all.proxy_arp_pvlan

Introduced in 65324144, the "proxy_arp_vlan" sysctl is a
per-interface sysctl to tune proxy ARP support for private VLANs.
While the "all" variant is exposed, it was a noop and never evaluated.
We use the usual "or" logic for this kind of sysctls.

Fixes: 65324144 ("net: RFC3069, private VLAN proxy arp support")
Signed-off-by: Vincent Bernat <vincent@bernat.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>