ipv6: create multicast route with RTPROT_KERNEL

ipv6: create multicast route with RTPROT_KERNEL

BugLink: https://bugs.launchpad.net/bugs/1915195
commit a826b04303a40d52439aa141035fca5654ccaccd upstream.

The ff00::/8 multicast route is created without specifying the fc_protocol
field, so the default RTPROT_BOOT value is used:

  $ ip -6 -d route
  unicast ::1 dev lo proto kernel scope global metric 256 pref medium
  unicast fe80::/64 dev eth0 proto kernel scope global metric 256 pref medium
  unicast ff00::/8 dev eth0 proto boot scope global metric 256 pref medium

As the documentation says, this value identifies routes installed during
boot, but the route is created when interface is set up.
Change the value to RTPROT_KERNEL which is a better value.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: William Breathitt Gray <william.gray@canonical.com>