NULL terminate fs driver options' list, validate_opt() looks for
a null entry to terminate the loop.

Fixes: aee7f3ec ("fsdev: Error out when unsupported option is passed")
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20200709175848.650400-1-ppandit@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>