mm: cma: Don't crash on allocation if CMA area can't be activated

If activation of the CMA area fails its mutex won't be initialized,
leading to an oops at allocation time when trying to lock the mutex. Fix
this by setting the cma area count field to 0 when activation fails,
leading to allocation returning NULL immediately.

Cc: <[email protected]>  # v3.17
Signed-off-by: Laurent Pinchart <[email protected]>
Acked-by: Michal Nazarewicz <[email protected]>
Signed-off-by: Marek Szyprowski <[email protected]>

mm/cma.c

@@ -124,6 +124,7 @@ static int __init cma_activate_area(struct cma *cma)
 
 err:
 	kfree(cma->bitmap);
+	cma->count = 0;
 	return -EINVAL;
 }