kcov: convert kcov.refcount to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:

 - counter is initialized to 1 using atomic_set()

 - a resource is freed upon counter reaching zero

 - once counter reaches zero, its further
   increments aren't allowed

 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided refcount_t
type and API that prevents accidental counter overflows and underflows.
This is important since overflows and underflows can lead to
use-after-free situation and be exploitable.

The variable kcov.refcount is used as pure reference counter.  Convert
it to refcount_t and fix up the operations.

**Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c have
different memory ordering guarantees than their atomic counterparts.

The full comparison can be seen in https://lkml.org/lkml/2017/11/15...