IPv6: reassembly: replace magic number with macro definitions

Use macro to define high/low thresh value, refer to IPV6_FRAG_TIMEOUT.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Patrick McHardy <kaber@trash.net>