GFS2: Add commit= mount option

It has always been possible to adjust the gfs2 log commit
interval, but only from the sysfs interface. This adds a
mount option, commit=<nn>, which will be familar to ext3
users.

The sysfs interface continues to be available as well, although
this might be removed in the future.

Also this patch cleans up some duplicated structures in the GFS2
sysfs code.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/incore.h

@@ -418,6 +418,7 @@ struct gfs2_args {
 	unsigned int ar_data:2;			/* ordered/writeback */
 	unsigned int ar_meta:1;			/* mount metafs */
 	unsigned int ar_discard:1;		/* discard requests */
+	int ar_commit;				/* Commit interval */
 };
 
 struct gfs2_tune {