stddef.h: move offsetofend inside #ifndef/#endif guard, neaten

Commit 38764884

 ("include/stddef.h: Move offsetofend() from vfio.h
to a generic kernel header") added offsetofend outside the normal
include #ifndef/#endif guard.  Move it inside.

Miscellanea:

o remove unnecessary blank line
o standardize offsetof macros whitespace style

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>