net: ip6mr: fix static mfc/dev leaks on table destruction

[ Upstream commit 4c6980462f32b4f282c5d8e5f7ea8070e2937725 ]

Similar to ipv4, when destroying an mrt table the static mfc entries and
the static devices are kept, which leads to devices that can never be
destroyed (because of refcnt taken) and leaked memory. Make sure that
everything is cleaned up on netns destruction.

Fixes: 8229efda ("netns: ip6mr: enable namespace support in ipv6 multicast forwarding code")
CC: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>