ext2, ext4: make mb block cache names more explicit

There will be a second mb_cache instance that tracks ea_inodes. Make
existing names more explicit so that it is clear that they refer to
xattr block cache.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext2/ext2.h b/fs/ext2/ext2.h
index 03f5ce1..23ebb92 100644
--- a/fs/ext2/ext2.h
+++ b/fs/ext2/ext2.h
@@ -113,7 +113,7 @@ struct ext2_sb_info {
 	 * of the mount options.
 	 */
 	spinlock_t s_lock;
-	struct mb_cache *s_mb_cache;
+	struct mb_cache *s_ea_block_cache;
 };
 
 static inline spinlock_t *