Revert "netfilter: conntrack: convert to refcount_t api"

This reverts commit bcba40bd.

It breaks the abi and is not really needed for Android systems, so
revert it for now.

Bug: 161946584
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I8768fa3c558d43cbe2005c151c494c9fd6146089