include/stddef.h: Move offsetofend() from vfio.h to a generic kernel header

Suggested by Andy.

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1425912738-559-1-git-send-email-dvlasenk@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>