ipv6: use seq_release_private for ip6mr.c /proc entries

In ip6mr.c, /proc entries /proc/net/ip6_mr_cache and /proc/net/ip6_mr_vif
are opened with seq_open_private(), thus seq_release_private() should be 
used to release them.
Should fix a small memory leak.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>