may be guards of ipv6_label/2 are wrong?.

[shun159]

ipv6_flabel is 20bit.
but byte_size(val) == 20, may should be bit_size(val) == 20.

https://github.com/FlowForwarding/of_msg_lib/blob/master/src/of_msg_lib_v4.erl#L914
https://github.com/FlowForwarding/of_msg_lib/blob/master/src/of_msg_lib_v5.erl#L1036

I fixed as follows:
https://github.com/shun159/of_msg_lib/blob/fix/ipv6_label_guard/src/of_msg_lib_v5.erl#L1036
https://github.com/shun159/of_msg_lib/blob/fix/ipv6_label_guard/src/of_msg_lib_v4.erl#L914

regards.