jbd2: fix invalid descriptor block checksum

In jbd2_journal_commit_transaction(), if we are in abort mode,
we may flush the buffer without setting descriptor block checksum
by goto start_journal_io. Then fs is mounted,
jbd2_descriptor_block_csum_verify() failed.

[  271.379811] EXT4-fs (vdd): shut down requested (2)
[  271.381827] Aborting journal on device vdd-8.
[  271.597136] JBD2: Invalid checksum recovering block 22199 in log
[  271.598023] JBD2: recovery failed
[  271.598484] EXT4-fs (vdd): error loading journal

Fix this problem by keep setting descriptor block checksum if the
descriptor buffer is not NULL.

This checksum problem can be reproduced by xfstests generic/388.
Signed-off-by: luojiajun <luojiajun3@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/jbd2/commit.c

@@ -694,9 +694,11 @@ void jbd2_journal_commit_transaction(journal_t *journal)
                            the last tag we set up. */
 
 			tag->t_flags |= cpu_to_be16(JBD2_FLAG_LAST_TAG);
-
-			jbd2_descriptor_block_csum_set(journal, descriptor);
 start_journal_io:
+			if (descriptor)
+				jbd2_descriptor_block_csum_set(journal,
+							descriptor);
+
 			for (i = 0; i < bufs; i++) {
 				struct buffer_head *bh = wbuf[i];
 				/*