cifs: reinstate the forcegid option

Apparently this was lost when we converted to the standard option
parser in 8830d7e0

Cc: Sachin Prabhu <sprabhu@redhat.com>
Cc: stable@vger.kernel.org # v3.4+
Reported-by: Gregory Lee Bartholomew <gregory.lee.bartholomew@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>