overflow: Introduce struct_offset() to get offset of member

overflow: Introduce struct_offset() to get offset of member

The trace_marker_raw file in tracefs takes a buffer from user space that
contains an id as well as a raw data string which is usually a binary
structure. The structure used has the following:

	struct raw_data_entry {
		struct trace_entry	ent;
		unsigned int		id;
		char			buf[];
	};

Since the passed in "cnt" variable is both the size of buf as well as the
size of id, the code to allocate the location on the ring buffer had:

   size = struct_size(entry, buf, cnt - sizeof(entry->id));

Which is quite ugly and hard to understand. Instead, add a helper macro
called struct_offset() which then changes the above to a simple and easy
to understand:

   size = struct_offset(entry, id) + cnt;

This will likely come in handy for other use cases too.

Link: https://lore.kernel.org/all/CAHk-=whYZVoEdfO1PmtbirPdBMTV9Nxt9f09CK0k6S+HJD3Zmg@mail.gmail.com/
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://patch.msgid.link/20251126145249.05b1770a@gandalf.local.home
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>