The goo.gl URL shortener is deprecated and is due to stop
expanding existing links in 2025.

The old goo.gl link in the 9p docs doesn't work anyway,
replace it by a kernel.org link suggested by Randy instead.
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240725180041.80862-1-linux@treblig.org