tracing: initcall: Ordered comparison of function pointers

Using initcall_t in the __field macro generates the following warning
with clang version 6.0:

include/trace/events/initcall.h:34:3: warning: ordered comparison of
function pointers ('initcall_t' (aka 'int (*)(void)') and 'initcall_t')

__field macro expands to __field_ext macro which does is_signed_type
check on the type argument. Since initcall_t is defined as a function
pointer, using it as the type in the __field macro, leads to an ordered
comparison of function pointer warning, inside the check. Using
__field_struct macro avoids the issue.

Link: http://lkml.kernel.org/r/1524699755-29388-1-git-send-email-rishabhb@codeaurora.org

Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>
[ Added comment to why we are using field_struct() ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/trace/events/initcall.h

@@ -31,7 +31,11 @@ TRACE_EVENT(initcall_start,
 	TP_ARGS(func),
 
 	TP_STRUCT__entry(
-		__field(initcall_t, func)
+		/*
+		 * Use field_struct to avoid is_signed_type()
+		 * comparison of a function pointer
+		 */
+		__field_struct(initcall_t, func)
 	),
 
 	TP_fast_assign(
@@ -48,8 +52,12 @@ TRACE_EVENT(initcall_finish,
 	TP_ARGS(func, ret),
 
 	TP_STRUCT__entry(
-		__field(initcall_t,	func)
-		__field(int,		ret)
+		/*
+		 * Use field_struct to avoid is_signed_type()
+		 * comparison of a function pointer
+		 */
+		__field_struct(initcall_t,	func)
+		__field(int,			ret)
 	),
 
 	TP_fast_assign(