tuntap: limit the number of flow caches

We create new flow caches when a new flow is identified by tuntap, This may lead
some issues:

- userspace may produce a huge amount of short live flows to exhaust host memory
- the unlimited number of flow caches may produce a long list which increase the
  time in the linear searching

Solve this by introducing a limit of total number of flow caches.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>