dm cache metadata: remove unnecessary variable in __dump_mapping

Fix the following coccicheck warning:

drivers/md/dm-cache-metadata.c:1512:5-6: Unneeded variable: "r".
Return "0" on line 1520.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-cache-metadata.c

@@ -1509,15 +1509,14 @@ int dm_cache_load_mappings(struct dm_cache_metadata *cmd,
 
 static int __dump_mapping(void *context, uint64_t cblock, void *leaf)
 {
-	int r = 0;
 	__le64 value;
 	dm_oblock_t oblock;
 	unsigned flags;
 
 	memcpy(&value, leaf, sizeof(value));
 	unpack_value(value, &oblock, &flags);
 
-	return r;
+	return 0;
 }
 
 static int __dump_mappings(struct dm_cache_metadata *cmd)