[NETNS][IPV6]: Make mld_max_msf readonly in other namespaces.

The mld_max_msf protects the system with a maximum allowed multicast
source filters. Making this variable per namespace can be potentially
an problem if someone inside a namespace set it to a big value, that
will impact the whole system including other namespaces.

I don't see any benefits to have it per namespace for now, so in order
to keep a directory entry in a newly created namespace, I make it
read-only when we are not in the initial network namespace.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>