ext4: fix a wrong comment in __mb_check_buddy()

The comment says the bit should be 0, but the after code assert the
bit to be 1.  This makes people confused, so fix it.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 2bb1ddc..e2d8be8 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -581,7 +581,7 @@
 				continue;
 			}
 
-			/* both bits in buddy2 must be 0 */
+			/* both bits in buddy2 must be 1 */
 			MB_CHECK_ASSERT(mb_test_bit(i << 1, buddy2));
 			MB_CHECK_ASSERT(mb_test_bit((i << 1) + 1, buddy2));