sys/linux: add basic examples of vusb variants

This change is the first step in addressing issue [1].

Despite syzkaller's best efforts, some usb drivers are proving
resistant to attempts to probe them. Specifically, crafted
devices are not accurate enough to bypass checks in probe().
These checks mostly deal with usb interfaces and endpoints.

One way to address this issue is to define syz_connect_* calls
variants to help syzkaller succeed probing by describing in detail
various device attributes.

Start by describing such calls for select drivers, each representing
its own category of sorts. At the moment, code coverage for these
drivers is unimpressive:

- rtl8150
Used to succeed probing until a better usb endpoint check was implemented.

- sierra_net
Same as rtl8150. Depends on usbnet API for bind() and usb ep checks.

- lan78xx
Requires numerous control requests between driver and device DURING probe.
Extra descriptions are helpful but are not enough to fully complete
probing process.

Also, add a seed for each such example.

This is only a stepping stone to improve usb fuzzing results and most
likely will be subject to change in the future.

[1] https://github.com/google/syzkaller/issues/6206

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