ovl: allow setting max size of redirect

Add a module option to allow tuning the max size of absolute redirects.
Default is 256.

Size of relative redirects is naturally limited by the the underlying
filesystem's max filename length (usually 255).
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/dir.c

@@ -12,12 +12,18 @@
 #include <linux/xattr.h>
 #include <linux/security.h>
 #include <linux/cred.h>
+#include <linux/module.h>
 #include <linux/posix_acl.h>
 #include <linux/posix_acl_xattr.h>
 #include <linux/atomic.h>
 #include <linux/ratelimit.h>
 #include "overlayfs.h"
 
+static unsigned short ovl_redirect_max = 256;
+module_param_named(redirect_max, ovl_redirect_max, ushort, 0644);
+MODULE_PARM_DESC(ovl_redirect_max,
+		 "Maximum length of absolute redirect xattr value");
+
 void ovl_cleanup(struct inode *wdir, struct dentry *wdentry)
 {
 	int err;
@@ -777,13 +783,11 @@ static bool ovl_can_move(struct dentry *dentry)
 		!d_is_dir(dentry) || !ovl_type_merge_or_lower(dentry);
 }
 
-#define OVL_REDIRECT_MAX 256
-
 static char *ovl_get_redirect(struct dentry *dentry, bool samedir)
 {
 	char *buf, *ret;
 	struct dentry *d, *tmp;
-	int buflen = OVL_REDIRECT_MAX + 1;
+	int buflen = ovl_redirect_max + 1;
 
 	if (samedir) {
 		ret = kstrndup(dentry->d_name.name, dentry->d_name.len,