tracing: initcall: Ordered comparison of function pointers

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>