From: Alex Tomas <bzzz@tmi.comex.ru>

fsync_super() calls ->sync_fs() just after ->write_super().  But
write_super() will start a commit.  In this case, ext3_sync_fs() will not
itself start a commit, and it hence forgets to wait on the commit which
ext3_write_super() started.

Fix that up by making journal_start_commit() return the transaction ID of
any currently-running transaction.