p2p: define DiscReason as uint8

[iczc]

cherry-pick form ethereum#24507 to fix CVE-2022-29177.

[ChuhanJin]

Can one of the admins verify this patch?

[0xmountaintop]

close and re-open to trigger CI

[iczc]

Since DiscReason was an alias type of unit, the malicious node can build discMsg with math.MaxUint DiscReason and send it to a victim node.
A victim node if configured to use high verbosity logging(>3) will print the DiscReason, then the String() method of the DiscReason type will be called.

func (d DiscReason) String() string {
	if len(discReasonToString) <= int(d) {
		return fmt.Sprintf("unknown disconnect reason %d", d)
	}
	return discReasonToString[d]
}

func (d DiscReason) Error() string {
	return d.String()
}

As int(math.MaxUint) results in -1, the if branch will not be executed, and an "index out of range" runtime error PANIC will occur on return discReasonToString[d], this can cause the node to crash.