ifconfig: fix incorrect wg allowed-ips netmask

Currently when peer information is displayed with ifconfig wgN peer ..
or ifconfig wgN peer-list, the netmask of the first allowed-ips will
be used as the netmask of all CIDR in allowed-ips. For example, if
the list is 192.168.1.0/24, 172.16.0.0/16, it will display as
192.168.1.0/24, 172.16.0.0/24. While this does not affect the actual
functionality, it is very confusing.

Submitted by: Michael Chiu <nyan -at- myuji.xyz>
Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D28655
MFC after: 1 day