net/9p: autoload transport modules

Automatically load transport modules based on the trans= parameter
passed to mount.
This removes the requirement for the user to know which module to use.

Link: http://lkml.kernel.org/r/20211017134611.4330-1-linux@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>