livepatch: return -ENOMEM on ptr_id() allocation failure

Fixes the following smatch warning:

  lib/livepatch/test_klp_shadow_vars.c:47 ptr_id() warn: returning -1 instead of -ENOMEM is sloppy
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Petr Mladek <pmladek@suse.com>

lib/livepatch/test_klp_shadow_vars.c

@@ -44,7 +44,7 @@ static int ptr_id(void *ptr)
 
 	sp = kmalloc(sizeof(*sp), GFP_ATOMIC);
 	if (!sp)
-		return -1;
+		return -ENOMEM;
 	sp->ptr = ptr;
 	sp->id = count++;