ptrace: cleanup arch_ptrace() on sh

Remove unnecessary castings and get rid of dummy pointer in favor of
offsetof() macro in ptrace_32.c. Also use temporary variables and
break long lines in order to improve readability.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>