net: flow_dissector: Use 64bits for used_keys

As 32bits of dissector->used_keys are exhausted,
increase the size to 64bits.

This is base change for ESP/AH flow dissector patch.
Please find patch and discussions at
https://lore.kernel.org/netdev/ZMDNjD46BvZ5zp5I@corigine.com/T/#t

Signed-off-by: Ratheesh Kannoth <rkannoth@marvell.com>
Reviewed-by: Petr Machata <petrm@nvidia.com> # for mlxsw
Tested-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Martin Habets <habetsm.xilinx@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>