mount: Don't call canonicalize_*(SPEC) for 9p

mount: Don't call canonicalize_*(SPEC) for 9p

When calling "mount -t 9p -o trans=virtio foobar /mnt/bar" and foobar
exists in the current path, the 9p virtio transport driver will be
called with $CWD/foobar and fail with "9p: no channels available".

Similar problems exist with remote file servers
 "mount -t 9p 23.42.08.15 /mnt/bar"
and Plan 9 From User Space applications
 "mount -t 9p -o trans=unix,uname=$USER `namespace`/acme /mnt/bar"

A similar exception like for nfs, cifs and smbfs must be added for 9p.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>