md-cache: Make options structure NULL terminated.

Change-Id: I8aa4f90ba7e1eecf3f978be04f8550049275464f
BUG: 765785
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/4994
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>