netifd: allow negative neighlocktime values

When -1 is written in /proc/sys/net/ipv4/neigh/<iface>/locktime,
kernel disables ARP trashing protection. A value of 0 does not completely
disable this protection, a second ARP update being discarded if it
is processed during the same jiffie as the first update.

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>