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

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

Commit 3876488444e7 ("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 <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>