fix: prevent panic when using diskstats device-include only (#3590)

[mumberrymountain]

this PR fix issue #3590.

Problem:
setting --collector.diskstats.device-include alone trigger the “device-exclude & device-include are mutually exclusive” error and ultimately cause panic during handler creation.

Cause:

diskstatsDeviceExclude    = kingpin.Flag(
		"collector.diskstats.device-exclude",
		"Regexp of diskstats devices to exclude (mutually exclusive to device-include).",
	).Default(diskstatsDefaultIgnoredDevices).PreAction(func(c *kingpin.ParseContext) error {
		diskstatsDeviceExcludeSet = true
		return nil
}).String()

collector.diskstats.device-exclude has a non-empty default on Linux,

if *diskstatsDeviceExclude != "" && *diskstatsDeviceInclude != "" {
		return deviceFilter{}, errors.New("device-exclude & device-include are mutually exclusive")
}

as a result, providing device-include only still leaves device-exclude as default, so the code treats it as include+exclude all being set and throws error.

Fix

if *diskstatsDeviceInclude != "" {
	if diskstatsDeviceExcludeSet {
		return deviceFilter{}, errors.New("device-exclude & device-include are mutually exclusive")
	}
	*diskstatsDeviceExclude = ""
}

When device-include is set:

• Return a mutual exclusivity error only if device-exclude was explicitly provided by the user
• Otherwise, ignore/clear the default exclude so include-only works as intended