ptrace: cleanup arch_ptrace() on powerpc

Use new 'datavp' and 'datalp' variables in order to remove unnecessary
castings.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>