ovl: fix regression in parsing of mount options with escaped comma

Ever since commit 91c77947 ("ovl: allow filenames with comma"), the
following example was legit overlayfs mount options:

  mount -t overlay overlay -o 'lowerdir=/tmp/a\,b/lower' /mnt

The conversion to new mount api moved to using the common helper
generic_parse_monolithic() and discarded the specialized ovl_next_opt()
option separator.

Bring back ovl_next_opt() and use vfs_parse_monolithic_sep() to fix the
regression.
Reported-by: Ryan Hendrickson <ryan.hendrickson@alum.mit.edu>
Closes: https://lore.kernel.org/r/8da307fb-9318-cf78-8a27-ba5c5a0aef6d@alum.mit.edu/
Fixes: 1784fbc2 ("ovl: port to new mount api")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

fs/overlayfs/params.c

@@ -157,6 +157,34 @@ const struct fs_parameter_spec ovl_parameter_spec[] = {
 	{}
 };
 
+static char *ovl_next_opt(char **s)
+{
+	char *sbegin = *s;
+	char *p;
+
+	if (sbegin == NULL)
+		return NULL;
+
+	for (p = sbegin; *p; p++) {
+		if (*p == '\\') {
+			p++;
+			if (!*p)
+				break;
+		} else if (*p == ',') {
+			*p = '\0';
+			*s = p + 1;
+			return sbegin;
+		}
+	}
+	*s = NULL;
+	return sbegin;
+}
+
+static int ovl_parse_monolithic(struct fs_context *fc, void *data)
+{
+	return vfs_parse_monolithic_sep(fc, data, ovl_next_opt);
+}
+
 static ssize_t ovl_parse_param_split_lowerdirs(char *str)
 {
 	ssize_t nr_layers = 1, nr_colons = 0;
@@ -682,6 +710,7 @@ static int ovl_reconfigure(struct fs_context *fc)
 }
 
 static const struct fs_context_operations ovl_context_ops = {
+	.parse_monolithic = ovl_parse_monolithic,
 	.parse_param = ovl_parse_param,
 	.get_tree    = ovl_get_tree,
 	.reconfigure = ovl_reconfigure,