Raise ValidationError on perm change for readonly paths

This commit introduces additional validation to check whether the
path on which user is trying to change permissions is set to
readonly. This appears to be a fairly common occurance with our
SCALE userbase and so we need to cleanly explain why they can't
do this.

src/middlewared/middlewared/plugins/filesystem_/acl.py

@@ -84,6 +84,13 @@ def _common_perm_path_validate(self, schema, data, verrors, pool_mp_ok=False):
                 f'{schema}.path',
                 'Path component for is currently encrypted and locked'
             )
+        else:
+            statfs_flags = self.middleware.call_sync('filesystem.statfs', path)['flags']
+            if 'RO' in statfs_flags:
+                verrors.add(
+                    f'{schema}.path',
+                    f'{path}: dataset underlying path has the readonly property enabled.'
+                )
 
         return loc