file-systems: Validate 'no-diratime flag.

This follows up on commit c077345539, and
adds a comment to avoid this in future.

* gnu/system/file-systems.scm (invalid-file-system-flags):
Add 'no-diratime to the list of KNOWN-FLAGS.

gnu/build/file-systems.scm

@@ -1123,6 +1123,7 @@ an exception in such cases but perform the nearest sane action."
   "Return the number suitable for the 'flags' argument of 'mount' that
 corresponds to the symbols listed in FLAGS."
   (let loop ((flags flags))
+    ;; Note: Keep in sync with ‘invalid-file-system-flags’.
     (match flags
       (('read-only rest ...)
        (logior MS_RDONLY (loop rest)))

gnu/system/file-systems.scm

@@ -122,7 +122,7 @@
     ;; Note: Keep in sync with 'mount-flags->bit-mask'.
     (let ((known-flags '(read-only
                          bind-mount no-suid no-dev no-exec
-                         no-atime strict-atime lazy-time
+                         no-atime no-diratime strict-atime lazy-time
                          shared)))
       (lambda (flags)
         "Return the subset of FLAGS that is invalid."