jbd2: fix FS corruption possibility in jbd2_journal_destroy() on umount path

On umount path, jbd2_journal_destroy() writes latest transaction ID
(->j_tail_sequence) to be used at next mount.

The bug is that ->j_tail_sequence is not holding latest transaction ID
in some cases. So, at next mount, there is chance to conflict with
remaining (not overwritten yet) transactions.

	mount (id=10)
	write transaction (id=11)
	write transaction (id=12)
	umount (id=10) <= the bug doesn't write latest ID

	mount (id=10)
	write transaction (id=11)
	crash

	mount
	[recovery process]
		transaction (id=11)
		transaction (id=12) <= valid transaction ID, but old commit
                                       must not replay

Like above, this bug become the cause of recovery failure, or FS
corruption.

So why ->j_tail_sequence doesn't point latest ID?

Because if checkpoint transactions was reclaimed by memory pressure
(i.e. bdev_try_to_free_page()), then ->j_tail_sequence is not updated.
(And another case is, __jbd2_journal_clean_checkpoint_list() is called
with empty transaction.)

So in above cases, ->j_tail_sequence is not pointing latest
transaction ID at umount path. Plus, REQ_FLUSH for checkpoint is not
done too.

So, to fix this problem with minimum changes, this patch updates
->j_tail_sequence, and issue REQ_FLUSH.  (With more complex changes,
some optimizations would be possible to avoid unnecessary REQ_FLUSH
for example though.)

BTW,

	journal->j_tail_sequence =
		++journal->j_transaction_sequence;

Increment of ->j_transaction_sequence seems to be unnecessary, but
ext3 does this.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org