Revert "netfilter: conntrack: convert to refcount_t api"

This reverts commit f483ed6e which is
commit 71977437 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I5910860c160ee55b79d2f219f6f4835499072abe
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>