net: ip6mr: add RTM_GETROUTE netlink op

The IPv6 multicast routing code previously implemented only the dump
variant of RTM_GETROUTE.  Implement single MFC item retrieval by copying
and adapting the respective IPv4 code.

Tested against FRRouting's IPv6 PIM stack.

Signed-off-by: David Lamparter <equinox@diac24.net>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>