cls_flow: remove one dynamic array

Its better to use a predefined size for this small automatic variable.

Removes a sparse error as well :

net/sched/cls_flow.c:288:13: error: bad constant expression

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>