mdstat: fix list detach issues

Move ent = ent->next; to while. It was outside the loop so if there
are more than 2 elements and we are looking for 3rd element it causes
infinite loop..

Fix el->next zeroing. It causes segfault in mdstat_free(). Theses
issues were not visible in my testing because I had only 2 MD devices.

Fixes: 4b3644a ("mdstat: Rework mdstat external arrays handling")
Signed-off-by: Mariusz Tkaczyk <[email protected]>

mdstat.c

@@ -123,13 +123,15 @@ static void mdstat_ent_list_detach_element(struct mdstat_ent **list_head, struct
 				ent->next = el->next;
 				break;
 			}
+
+			ent = ent->next;
 		}
 
-		ent = ent->next;
 	}
 
+	/* Guard if not found or list is empty - not allowed */
 	assert(ent);
-	ent->next = NULL;
+	el->next = NULL;
 }
 
 void free_mdstat(struct mdstat_ent *ms)