9p: re-enable mount time debug option

During reorganization, the mount time debug option was removed in favor
of module-load-time parameters.  However, the mount time option is still
a useful for feature during debug and for user-fault isolation when the
module is compiled into the kernel.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>
diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index 4feb5ae..45c3598 100644
--- a/fs/9p/v9fs.c
+++ b/fs/9p/v9fs.c
@@ -43,7 +43,7 @@
 
 enum {
 	/* Options that take integer arguments */
-	Opt_port, Opt_msize, Opt_uid, Opt_gid, Opt_afid,
+	Opt_debug, Opt_port, Opt_msize, Opt_uid, Opt_gid, Opt_afid,
 	Opt_rfdno, Opt_wfdno,
 	/* String options */
 	Opt_uname, Opt_remotename,
@@ -56,6 +56,7 @@
 };
 
 static match_table_t tokens = {
+	{Opt_debug, "debug=%x"},
 	{Opt_port, "port=%u"},
 	{Opt_msize, "msize=%u"},
 	{Opt_uid, "uid=%u"},
@@ -128,6 +129,10 @@
 			}
 		}
 		switch (token) {
+		case Opt_debug:
+			v9ses->debug = option;
+			p9_debug_level = option;
+			break;
 		case Opt_port:
 			v9ses->port = option;
 			break;