can: avoids a false warning

At this point optlen == sizeof(sfilter) but some compilers are dumb.

Reported-by: Németh Márton <nm127@freemail.h>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Oliver Hartkopp <oliver@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>