get parameter checks consistent with sysfs

The module can be loaded with arp_ip_target="255.255.255.255" which makes
 it impossible to remove as the function in sysfs checks for that value,
 so we make the parameter checks consistent with sysfs.

 v2: Fix formatting
 v3: Make description text < 75 columns

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
W