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: NRobin Dong <sanbai@taobao.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -581,7 +581,7 @@ static int __mb_check_buddy(struct ext4_buddy *e4b, char *file,
 				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));