[POWERPC] Use FIELD_SIZEOF in arch/ppc

Robert P.J. Day proposed to use the macro FIELD_SIZEOF in replace of code
that matches its definition.

The modification was made using the following semantic patch
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@haskernel@
@@

#include <linux/kernel.h>

@depends on haskernel@
type t;
identifier f;
@@

- (sizeof(((t*)0)->f))
+ FIELD_SIZEOF(t, f)

@depends on haskernel@
type t;
identifier f;
@@

- sizeof(((t*)0)->f)
+ FIELD_SIZEOF(t, f)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Paul Mackerras <paulus@samba.org>