ipv6: ip6mr: remove net pointer from struct mfc6_cache

ipv6: ip6mr: remove net pointer from struct mfc6_cache

Now that cache entries in unres_queue don't need to be distinguished by their
network namespace pointer anymore, we can remove it from struct mfc6_cache
add pass the namespace as function argument to the functions that need it.

Signed-off-by: Patrick McHardy <kaber@trash.net>