[NET_SCHED]: cls_flow: fix key mask validity check

Since we're using fls(), we need to check whether the value is
non-zero first.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/sched/cls_flow.c
Simple merge