fix(#139): do not check for host endianness in IP conversion

[rectified95]

Description

Function determineEndian() always returns the same value since the host byte order is always little-endian, so it can be skipped.
What's more, the byte order of IP address values passed by eBPF programs doesn't need to be changed, because Golang's net.IP type expects big-endian byte arrays.

Details:

We always enter this if-branch binary.LittleEndian.PutUint32(ip, nn) which looks like it performs conversion TO little-endian, while it actually expects to receive input already IN little-endian.
Note, the input number's bytes are kept in the same order - this function merely converts a uint32 to a byte array. The result is we convert a big-endian number to a big-endian byte array.

func (littleEndian) PutUint32(b []byte, v uint32) {
	_ = b[3] // early bounds check to guarantee safety of writes below
	b[0] = byte(v)
	b[1] = byte(v >> 8)
	b[2] = byte(v >> 16)
	b[3] = byte(v >> 24)
}

Crucially, net.IP expects big-endian byte arrays. For example, IP 15.14.0.0 (0x0F0E0000 in host order, and 0x00000E0F in network order) should be passed in as [0F, 0E, 00, 00] (most significant byte at the lowest memory address) and so doesn't need inverting. Example here: https://play.golang.com/p/ykiP6mLB_gG

In summary, we currently do not perform byte order conversion and shouldn't start to, either in GO or eBPF code.

Related Issue

Fixes #139

This PR looks almost identical to #161 which was rejected because it didn't include byte order conversion in eBPF, but turns out that's not needed. It also uses the correct uint-to-array conversion functions.

Checklist

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• I have correctly attributed the author(s) of the code.
• I have tested the changes locally.
• I have followed the project's style guidelines.
• I have updated the documentation, if necessary.
• I have added tests, if applicable.

[nddq]

Relates to #95