Documentation/fs/9p: Expand goo.gl link

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