RAS/CEC: Correct ce_add_elem()'s returned values

ce_add_elem() uses different return values to signal a result from
adding an element to the collector. Commit in Fixes: broke the case
where the element being added is not found in the array. Correct that.

 [ bp: Rewrite commit message, add kernel-doc comments. ]

Fixes: de0e0624d8 ("RAS/CEC: Check count_threshold unconditionally")
Signed-off-by: William Roche <william.roche@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/1617722939-29670-1-git-send-email-william.roche@oracle.com

drivers/ras/cec.c

@@ -309,11 +309,20 @@ static bool sanity_check(struct ce_array *ca)
 	return ret;
 }
 
+/**
+ * cec_add_elem - Add an element to the CEC array.
+ * @pfn:	page frame number to insert
+ *
+ * Return values:
+ * - <0:	on error
+ * -  0:	on success
+ * - >0:	when the inserted pfn was offlined
+ */
 static int cec_add_elem(u64 pfn)
 {
 	struct ce_array *ca = &ce_arr;
+	int count, err, ret = 0;
 	unsigned int to = 0;
-	int count, ret = 0;
 
 	/*
 	 * We can be called very early on the identify_cpu() path where we are
@@ -330,8 +339,8 @@ static int cec_add_elem(u64 pfn)
 	if (ca->n == MAX_ELEMS)
 		WARN_ON(!del_lru_elem_unlocked(ca));
 
-	ret = find_elem(ca, pfn, &to);
-	if (ret < 0) {
+	err = find_elem(ca, pfn, &to);
+	if (err < 0) {
 		/*
 		 * Shift range [to-end] to make room for one more element.
 		 */