compress: Change variable name

'writen' and 'total_writen' variables has been defined in compress
xlator. This patch replaced those with proper name.

Change-Id: If8e38fae8f997a92686a891d64cbaf6b0b69129a
BUG: 1075417
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/8268
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Tested-by: Venky Shankar <vshankar@redhat.com>