ocfs2: Support commit= mount option

Mostly taken from ext3. This allows the user to set the jbd commit interval,
in seconds. The default of 5 seconds stays the same, but now users can
easily increase the commit interval. Typically, this would be increased in
order to benefit performance at the expense of data-safety.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

Documentation/filesystems/ocfs2.txt

@@ -61,3 +61,14 @@ data=writeback		Data ordering is not preserved, data may be written
 preferred_slot=0(*)	During mount, try to use this filesystem slot first. If
 			it is in use by another node, the first empty one found
 			will be chosen. Invalid values will be ignored.
+commit=nrsec	(*)	Ocfs2 can be told to sync all its data and metadata
+			every 'nrsec' seconds. The default value is 5 seconds.
+			This means that if you lose your power, you will lose
+			as much as the latest 5 seconds of work (your
+			filesystem will not be damaged though, thanks to the
+			journaling).  This default value (or any low value)
+			will hurt performance, but it's good for data-safety.
+			Setting it to 0 will have the same effect as leaving
+			it at the default (5 seconds).
+			Setting it to very large values will improve
+			performance.