Template registration problem in netflow9 and ipfix packages

[satoukick]

Hi, there are some mistakes, like the codes below, that will end up registering the same template to a session.

netflow/netflow9/packet.go

Lines 96 to 98 in 8c13bb0

	for _, tr := range tfs.Records {
	tr.register(s)
	}

netflow/netflow9/packet.go

Lines 130 to 132 in 8c13bb0

	for _, record := range ofs.Records {
	record.register(s)
	}

netflow/ipfix/packet.go

Lines 112 to 114 in 8c13bb0

	for _, record := range ots.Records {
	record.register(s)
	}

netflow/ipfix/packet.go

Lines 100 to 102 in 8c13bb0

	for _, tr := range ts.Records {
	tr.register(s)
	}

The problem is described below:

prior to Go 1.22, every freshly-declared loop variable used in a for loop is shared by all iterations during executing the loop. 

I find this blog very helpful, please check!

I think there are two ways to fix the problem:

1. Add go.mod and declare go 1.22 or later. This will fix the problem without modifying any existing codes.
   Here is the official Go blog for the problem fix: https://go.dev/blog/loopvar-preview

2. Make a record copy in the loop and register the copy to session. This may be a better choice for backwards compatibility.

Thank you!