tools: ynl: Initialise fixed headers to 0 in genetlink-legacy

This eliminates the need for e.g. --json '{"dp-ifindex":0}' which is not
too big a deal for ovs but will get tiresome for fixed header structs that
have many members.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/ynl.py

@@ -541,7 +541,7 @@ class YnlFamily(SpecFamily):
         if op.fixed_header:
             fixed_header_members = self.consts[op.fixed_header].members
             for m in fixed_header_members:
-                value = vals.pop(m.name)
+                value = vals.pop(m.name) if m.name in vals else 0
                 format = NlAttr.get_format(m.type, m.byte_order)
                 msg += format.pack(value)
         for name, value in vals.items():