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>

init, tracing: Add initcall trace events

Being able to trace the start and stop of initcalls is useful to see where
the timings are an issue. There is already an "initcall_debug" parameter,
but that can cause a large overhead itself, as the printing of the
information may take longer than the initcall functions.

Adding in a start and finish trace event around the initcall functions, as
well as a trace event that records the level of the initcalls, one can get a
much finer measurement of the times and interactions of the initcalls
themselves, as trace events are much lighter than printk()s.

Suggested-by: Abderrahmane Benbachir <abderrahmane.benbachir@polymtl.ca>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>