core: remove GLUSTERFS_CREATE_MODE_KEY usage

Change-Id: I23b8cb7223b91a55af1cd4214f61bbe0e87351f6
BUG: 952029
Signed-off-by: Amar Tumballi <amarts@redhat.com>
Reviewed-on: http://review.gluster.org/5683
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>