9p: fix debug compilation error

s/9p/v9fs.c: In function 'v9fs_parse_options':
fs/9p/v9fs.c:134: error: 'p9_debug_level' undeclared (first use in this function)

Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>

fs/9p/v9fs.c

@@ -131,7 +131,9 @@ static void v9fs_parse_options(char *options, struct v9fs_session_info *v9ses)
 		switch (token) {
 		case Opt_debug:
 			v9ses->debug = option;
+#ifdef CONFIG_NET_9P_DEBUG
 			p9_debug_level = option;
+#endif
 			break;
 		case Opt_port:
 			v9ses->port = option;