Link MTU option ignored in IPv6 router advertisements

[Chuck Anderson <cra WPI EDU>]

I'm not sure if this is a kernel thing or a NetworkManager thing.  Did
something change in how IPv6 router advertisements are handled by
NetworkManager in Fedora 20?

On my Fedora 20 system, the Link MTU option recevied from IPv6 router
advertisements is not being applied to the interface configuration:

kernel-3.16.2-200.fc20.x86_64
NetworkManager-0.9.9.0-44.git20131003.fc20.x86_64

> cat /proc/sys/net/ipv6/conf/wlp2s0/mtu 
1500

On my Fedora 19 system, this is working:

kernel-3.14.18-100.fc19.x86_64
NetworkManager-0.9.8.8-2.fc19.x86_64

> cat /proc/sys/net/ipv6/conf/p4p1/mtu
1480

In both cases, the RA has the Link MTU option:

> sudo rdisc6 -1 wlp2s0
Soliciting ff02::2 (ff02::2) on wlp2s0...

Hop limit                 :           64 (      0x40)
Stateful address conf.    :           No
Stateful other conf.      :           No
Mobile home agent         :           No
Router preference         :       medium
Neighbor discovery proxy  :           No
Router lifetime           :          300 (0x0000012c) seconds
Reachable time            :  unspecified (0x00000000)
Retransmit time           :  unspecified (0x00000000)
 Prefix                   : 2001:xxxx:xxxx:1::1/64
  On-link                 :          Yes
  Autonomous address conf.:          Yes
  Valid time              :          300 (0x0000012c) seconds
  Pref. time              :          300 (0x0000012c) seconds
 MTU                      :         1480 bytes (valid)
 Source link-layer address: 68:7F:74:xx:xx:xx
 from fe80::6a7f:74ff:fexx:xxxx

I need this option because my IPv6 router uses a HE.net tunnel which
adds 20 bytes of overhead for the 6in4 encapsulation, and many sites
break PMTUD which causes those sites to hang.