9p: fix bad kconfig cross-dependency

This patch moves transport dynamic registration and matching to the net
module to prevent a bad Kconfig dependency between the net and fs 9p modules.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

include/net/9p/transport.h

@@ -50,5 +50,7 @@ struct p9_trans_module {
 };
 
 void v9fs_register_trans(struct p9_trans_module *m);
+struct p9_trans_module *v9fs_match_trans(const substring_t *name);
+struct p9_trans_module *v9fs_default_trans(void);
 
 #endif /* NET_9P_TRANSPORT_H */