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>