Description of problem:
In IPv6, snmp trap packet does not send from specified interface assigned by clientaddr option in snmpd.conf.

Version-Release number of selected component (if applicable):
net-snmp-5.7.2-18.el7

How reproducible:
Always.

Steps to Reproduce:
1. Prepare two nics for the server.
2. Assign IPv6 address to two NICs.
e.x.
~~~
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 52:54:00:e1:9e:6c brd ff:ff:ff:ff:ff:ff
    inet6 2001:db4::101/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::5054:ff:fee1:9e6c/64 scope link 
       valid_lft forever preferred_lft forever
3: ens7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 52:54:00:61:3e:3e brd ff:ff:ff:ff:ff:ff
    inet6 2001:db4::1/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::5054:ff:fe61:3e3e/64 scope link 
       valid_lft forever preferred_lft forever
~~~

3. Assigned clientaddr to ens3
- /etc/snmp/snmpd.conf
~~~
clientaddr 2001:db4::101
~~~

4. Send snmp trap to another server.
5. Check source address in the packet.

Actual results:
The source address is not the assigned address.

Expected results:
The source address is the assigned address in clientaddr option.

Additional info:
This bug is reported from NEC. And upstream has a fix for this issue.

http://sourceforge.net/p/net-snmp/bugs/2584/