dm: remove unneeded variable 'sz'

Fix the following coccicheck warning:
./drivers/md/dm-ps-service-time.c:85:10-12: Unneeded variable: "sz".
Return "0" on line 105

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>

drivers/md/dm-ps-service-time.c

@@ -82,7 +82,6 @@ static void st_destroy(struct path_selector *ps)
 static int st_status(struct path_selector *ps, struct dm_path *path,
 		     status_type_t type, char *result, unsigned maxlen)
 {
-	unsigned sz = 0;
 	struct path_info *pi;
 
 	if (!path)
@@ -102,7 +101,7 @@ static int st_status(struct path_selector *ps, struct dm_path *path,
 		}
 	}
 
-	return sz;
+	return 0;
 }
 
 static int st_add_path(struct path_selector *ps, struct dm_path *path,