[media] vb2: convert vb2_vmarea_handler refcount from atomic_t to refcount_t

Use refcount_t to manage the refcount to the memory type specific buffer
videobuf2 buffer implementations. refcount_t is better suitable for the
purpose than atomic_t.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>