jbd2: Use round-jiffies() function for the "5 second" ext4/jbd2 wakeup

While "every 5 seconds" doesn't sound as a problem, there can be many
of these (and these timers do add up over all the kernel).  The "5
second" wakeup isn't really timing sensitive; in addition even with
rounding it'll still happen every 5 seconds (with the exception of the
very first time, which is likely to be rounded up to somewhere closer
to 6 seconds)

(Ported from similar JBD patch made by Arjan van de Ven to
fs/jbd/transaction.c)

Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/transaction.c

@@ -54,7 +54,7 @@ jbd2_get_transaction(journal_t *journal, transaction_t *transaction)
 	spin_lock_init(&transaction->t_handle_lock);
 
 	/* Set up the commit timer for the new transaction. */
-	journal->j_commit_timer.expires = transaction->t_expires;
+	journal->j_commit_timer.expires = round_jiffies(transaction->t_expires);
 	add_timer(&journal->j_commit_timer);
 
 	J_ASSERT(journal->j_running_transaction == NULL);