net: openvswitch: set max limitation to meters

Don't allow user to create meter unlimitedly, which may cause
to consume a large amount of kernel memory. The max number
supported is decided by physical memory and 20K meters as default.

Cc: Pravin B Shelar <pshelar@ovn.org>
Cc: Andy Zhou <azhou@ovn.org>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>