ynl-gen-cpp: add support for generating nlctrl

Summary:
Enable nlctrl code generation. and fix compile error
ynl_attr_data() return void* type which could not be implicitly cast to const struct nlattr* in C++. add explicit cast in code generation

Test Plan:
make sure code could be generated and compiled

Signed-off-by: Sheldon Wu <xzw2048@gmail.com>

Author: sheldnwu

generated/Makefile

@@ -17,7 +17,7 @@ GENS_PATHS=$(shell grep -nrI --files-without-match \
 		'protocol: netlink' ../Documentation/netlink/specs/)
 GENS_PATHS+=$(wildcard ../Documentation/netlink/specs/rt-*.yaml)
 GENS_ALL=$(patsubst ../Documentation/netlink/specs/%.yaml,%,${GENS_PATHS})
-GENS=$(filter-out nlctrl,${GENS_ALL})
+GENS=$(filter-out ,${GENS_ALL})
 SRCS=$(patsubst %,%-user.cpp,${GENS})
 HDRS=$(patsubst %,%-user.hpp,${GENS})
 OBJS=$(patsubst %,%-user.cpp.o,${GENS})

generated/nlctrl-user.cpp

@@ -522,7 +522,7 @@ int nlctrl_getpolicy_rsp_parse(const struct nlmsghdr *nlh,
 
 			parg.rsp_policy = &nlctrl_op_policy_attrs_nest;
 			parg.data = &dst->op_policy.emplace();
-			attr_op_id = ynl_attr_data(attr);
+			attr_op_id = (const struct nlattr*)ynl_attr_data(attr);
 			op_id = ynl_attr_type(attr_op_id);
 			nlctrl_op_policy_attrs_parse(&parg, attr_op_id, op_id);
 		} else if (type == CTRL_ATTR_POLICY) {
@@ -535,9 +535,9 @@ int nlctrl_getpolicy_rsp_parse(const struct nlmsghdr *nlh,
 
 			parg.rsp_policy = &nlctrl_policy_attrs_nest;
 			parg.data = &dst->policy.emplace();
-			attr_policy_id = ynl_attr_data(attr);
+			attr_policy_id = (const struct nlattr*)ynl_attr_data(attr);
 			policy_id = ynl_attr_type(attr_policy_id);
-			attr_attr_id = ynl_attr_data(attr_policy_id);
+			attr_attr_id = (const struct nlattr*)ynl_attr_data(attr_policy_id);
 			attr_id = ynl_attr_type(attr_attr_id);
 			nlctrl_policy_attrs_parse(&parg, attr_attr_id, policy_id, attr_id);
 		}

ynl-gen-cpp.py

@@ -813,7 +813,7 @@ def _attr_get(self, ri, var):
             local_vars += [f'__u32 {", ".join(tv_names)};']
             for level in self.attr["type-value"]:
                 level = c_lower(level)
-                get_lines += [f"attr_{level} = ynl_attr_data({prev});"]
+                get_lines += [f"attr_{level} = (const struct nlattr*)ynl_attr_data({prev});"]
                 get_lines += [f"{level} = ynl_attr_type(attr_{level});"]
                 prev = "attr_" + level