zcip: fix link-local IP conflict detection

During link-local IP resolution, if a regular ARP request is received
during the ARP probe period, it will incorrectly cause a target IP
conflict.  This then leads to a new IP being picked unnecessarily.

Per RFC 3927, section 2.2.1, we should flag a target IP conflict only if
the source IP is null, the target IP matches our IP, and the source
hw addr does not match our hw addr.

function                                             old     new   delta
zcip_main                                           1354    1322     -32

Signed-off-by: Ken Sharp <ken.sharp@ni.com>
Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>