if: fix same connected address

if: fix same connected address

Using the same address with a different prefix length is not supported.
If we configure two identical addresses with different
netmasks 192.168.1.1/30 and then 192.168.1.1/29. Zebra sends
'192.168.1.1' with a prefix length of 29. However, the function
'zebra_interface_address_read()' reads '192.168.1.1/30' because the
prefix length is not checked.

Using 'same_prefix()' is more convenient.

Signed-off-by: Loïc Sang <loic.sang@6wind.com>