Fix: check reference counts for overflow

Linux kernel CVE-2016-0728 is a use-after-free based on overflow of the
reference counting mechanism.

Implement a kref wrapper in lttng that validates overflows, and use it
instead of kref_get(). Also check explicitly for overflows on file
fcount counters.

This should not be an issue in practice in lttng-modules because the ABI
is only exposed to root, but let's err on the safe side.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>