debugfs show actual source in /proc/mounts

After its conversion to the new mount API, debugfs displays "none" in
/proc/mounts instead of the actual source.  Fix this by recognising its
"source" mount option.
Signed-off-by: Marc Aurèle La France <tsi@tuyoix.net>
Link: https://lore.kernel.org/r/e439fae2-01da-234b-75b9-2a7951671e27@tuyoix.net
Fixes: a20971c1 ("vfs: Convert debugfs to use the new mount API")
Cc: stable@vger.kernel.org # 6.10.x: 49abee59: debugfs: Convert to new uid/gid option parsing helpers
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/debugfs/inode.c

@@ -89,12 +89,14 @@ enum {
 	Opt_uid,
 	Opt_gid,
 	Opt_mode,
+	Opt_source,
 };
 
 static const struct fs_parameter_spec debugfs_param_specs[] = {
 	fsparam_gid	("gid",		Opt_gid),
 	fsparam_u32oct	("mode",	Opt_mode),
 	fsparam_uid	("uid",		Opt_uid),
+	fsparam_string	("source",	Opt_source),
 	{}
 };
 
@@ -126,6 +128,12 @@ static int debugfs_parse_param(struct fs_context *fc, struct fs_parameter *param
 	case Opt_mode:
 		opts->mode = result.uint_32 & S_IALLUGO;
 		break;
+	case Opt_source:
+		if (fc->source)
+			return invalfc(fc, "Multiple sources specified");
+		fc->source = param->string;
+		param->string = NULL;
+		break;
 	/*
 	 * We might like to report bad mount options here;
 	 * but traditionally debugfs has ignored all mount options