netfilter: nf_conntrack: fix lifetime display for disabled connections

When no tstamp extension exists, ct_delta_time() returns -1, which is
then assigned to an u64 and tested for negative values to decide
whether to display the lifetime. This obviously doesn't work, use
a s64 and merge the two minor functions into one.

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