ptrace: cleanup arch_ptrace() on um

Remove unnecessary castings using void pointer and fix copy_to_user()
return value. Also add missing __user markup on the argument of
arch_ptrctl().

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>