fix #3393: tools/xattr: allow xattr 'security.NTACL'

in some configurations, samba stores NTFS-ACLs in this xattr[0], so
we should backup (if we can)

altough the 'security' namespace is special (e.g. in use by
selinux, etc.) this value is normally only used by samba and we
should be able to back it up.

to restore it, the user needs at least 'CAP_SYS_ADMIN' rights, otherwise
it cannot be set

0: https://www.samba.org/samba/docs/current/man-html/vfs_acl_xattr.8.html

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/tools/xattr.rs

@@ -170,6 +170,10 @@ pub fn is_valid_xattr_name(c_name: &CStr) -> bool {
     if name.starts_with(b"user.") || name.starts_with(b"trusted.") {
         return true;
     }
+    // samba saves windows ACLs there
+    if name == b"security.NTACL" {
+        return true;
+    }
     is_security_capability(c_name)
 }