zerofill: Update API version
version 6 adds zerofill FOP

BUG: 1028673
Change-Id: I27cfc48cd6f7f0f6daf94e1c9cfbe420a0d090af
Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Reviewed-on: http://review.gluster.org/6255
Reviewed-by: Bharata B Rao <bharata.rao@gmail.com>
Tested-by: Bharata B Rao <bharata.rao@gmail.com>
Reviewed-by: Anand Avati <avati@redhat.com>