tools/bpftool: Make skeleton code C++17-friendly by dropping typeof()

Seems like C++17 standard mode doesn't recognize typeof() anymore. This can
be tested by compiling test_cpp test with -std=c++17 or -std=c++1z options.
The use of typeof in skeleton generated code is unnecessary, all types are
well-known at the time of code generation, so remove all typeof()'s to make
skeleton code more future-proof when interacting with C++ compilers.

Fixes: 985ead4 ("bpftool: Add skeleton codegen command")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200812025907.1371956-1-andriin@fb.com

tools/bpf/bpftool/gen.c

@@ -397,7 +397,7 @@ static int do_skeleton(int argc, char **argv)
 		{							    \n\
 			struct %1$s *obj;				    \n\
 									    \n\
-			obj = (typeof(obj))calloc(1, sizeof(*obj));	    \n\
+			obj = (struct %1$s *)calloc(1, sizeof(*obj));	    \n\
 			if (!obj)					    \n\
 				return NULL;				    \n\
 			if (%1$s__create_skeleton(obj))			    \n\
@@ -461,7 +461,7 @@ static int do_skeleton(int argc, char **argv)
 		{							    \n\
 			struct bpf_object_skeleton *s;			    \n\
 									    \n\
-			s = (typeof(s))calloc(1, sizeof(*s));		    \n\
+			s = (struct bpf_object_skeleton *)calloc(1, sizeof(*s));\n\
 			if (!s)						    \n\
 				return -1;				    \n\
 			obj->skeleton = s;				    \n\
@@ -479,7 +479,7 @@ static int do_skeleton(int argc, char **argv)
 				/* maps */				    \n\
 				s->map_cnt = %zu;			    \n\
 				s->map_skel_sz = sizeof(*s->maps);	    \n\
-				s->maps = (typeof(s->maps))calloc(s->map_cnt, s->map_skel_sz);\n\
+				s->maps = (struct bpf_map_skeleton *)calloc(s->map_cnt, s->map_skel_sz);\n\
 				if (!s->maps)				    \n\
 					goto err;			    \n\
 			",
@@ -515,7 +515,7 @@ static int do_skeleton(int argc, char **argv)
 				/* programs */				    \n\
 				s->prog_cnt = %zu;			    \n\
 				s->prog_skel_sz = sizeof(*s->progs);	    \n\
-				s->progs = (typeof(s->progs))calloc(s->prog_cnt, s->prog_skel_sz);\n\
+				s->progs = (struct bpf_prog_skeleton *)calloc(s->prog_cnt, s->prog_skel_sz);\n\
 				if (!s->progs)				    \n\
 					goto err;			    \n\
 			",