Sanitize the type of struct user.u_ar0

Sanitize the type of struct user.u_ar0

struct user.u_ar0 is defined to contain a pointer offset on all
architectures in which it is defined (all architectures which define an
a.out format except SPARC.) However, it has a pointer type in the headers,
which is pointless -- <asm/user.h> is not exported to userspace, and it
just makes the code messy.

Redefine the field as "unsigned long" (which is the same size as a pointer
on all Linux architectures) and change the setting code to user offsetof()
instead of hand-coded arithmetic.

Cc: Linux Arch Mailing List <[email protected]>
Cc: Bryan Wu <[email protected]>
Cc: Roman Zippel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Russell King <[email protected]>
Cc: Lennert Buytenhek <[email protected]>
Cc: Håvard Skinnemoen <[email protected]>
Cc: Mikael Starvik <[email protected]>
Cc: Yoshinori Sato <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Hirokazu Takata <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Paul Mundt <[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>