ext4: disable synchronous transaction batching if max_batch_time==0

commit 5dd214248f94d430d70e9230bda72f2654ac88a8 upstream.

The mount manpage says of the max_batch_time option,

	This optimization can be turned off entirely
	by setting max_batch_time to 0.

But the code doesn't do that.  So fix the code to do
that.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.2: option parsing looks different]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>