jbd2: fix potential memory leak on transaction commit

There is potential memory leak of journal head in function
jbd2_journal_commit_transaction. The problem is that JBD2 will not
reclaim the journal head of commit record if error occurs or journal
is abotred.

I use the following script to reproduce this issue, on a RHEL6
system. I found it very easy to reproduce with async commit enabled.

mount /dev/sdb /mnt -o journal_checksum,journal_async_commit
touch /mnt/xxx
echo offline > /sys/block/sdb/device/state
sync
umount /mnt
rmmod ext4
rmmod jbd2

Removal of the jbd2 module will make slab complaining that
"cache `jbd2_journal_head': can't free all objects".

Signed-off-by: Zhang Huan <zhhuan@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/commit.c

@@ -105,6 +105,8 @@ static int journal_submit_commit_record(journal_t *journal,
 	int ret;
 	struct timespec now = current_kernel_time();
 
+	*cbh = NULL;
+
 	if (is_journal_aborted(journal))
 		return 0;
 
@@ -806,7 +808,7 @@ wait_for_iobuf:
 		if (err)
 			__jbd2_journal_abort_hard(journal);
 	}
-	if (!err && !is_journal_aborted(journal))
+	if (cbh)
 		err = journal_wait_on_commit_record(journal, cbh);
 	if (JBD2_HAS_INCOMPAT_FEATURE(journal,
 				      JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT) &&