tools: ynl: fix render-max for flags definition

Properly manage render-max property for flags definition type
introducing mask value and setting it to (last_element << 1) - 1
instead of adding max value set to last_element + 1

Fixes: be5bea1 ("net: add basic C code generators for Netlink")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-c.py

@@ -1931,9 +1931,14 @@ def render_uapi(family, cw):
 
             if const.get('render-max', False):
                 cw.nl()
-                max_name = c_upper(name_pfx + 'max')
-                cw.p('__' + max_name + ',')
-                cw.p(max_name + ' = (__' + max_name + ' - 1)')
+                if const['type'] == 'flags':
+                    max_name = c_upper(name_pfx + 'mask')
+                    max_val = f' = {enum.get_mask()},'
+                    cw.p(max_name + max_val)
+                else:
+                    max_name = c_upper(name_pfx + 'max')
+                    cw.p('__' + max_name + ',')
+                    cw.p(max_name + ' = (__' + max_name + ' - 1)')
             cw.block_end(line=';')
             cw.nl()
         elif const['type'] == 'const':