[LINUX] ipv6: Disable addrconf on Xen bridge device

[LINUX] ipv6: Disable addrconf on Xen bridge device

The ipv6-no-autoconf patch didn't disable IPv6 addrconf completely.
This means that the Xen bridge device still interfered with normal
IPv6 operation by engaging the IPv6 network with a bogus MAC address.

For details please refer to
        https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=200360

The following patch completely disables IPv6 on the Xen bridge device
by temporarily setting the MTU to a value less than the minimum
allowed for IPv6.

Upstream will provide a cleaner way to disable IPv6 addrconf in
future, possibly in the form of a proc sysctl.  Of course if the Xen
loopback device is removed it would render this change unnecessary.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>