9pfs: drop unused fmt strings in the proxy backend

The v9fs_request() function doesn't use its fmt argument: it passes literal
format strings to proxy_marshal() for all commands.

This patch simply drops the unused fmt argument and updates all callers
accordingly.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>