um: Fix PTRACE_POKEUSER on x86_64

This is broken since ever but sadly nobody noticed.
Recent versions of GDB set DR_CONTROL unconditionally and
UML dies due to a heap corruption. It turns out that
the PTRACE_POKEUSER was copy&pasted from i386 and assumes
that addresses are 4 bytes long.

Fix that by using 8 as address size in the calculation.

Cc: <stable@vger.kernel.org>
Reported-by: jie cao <cj3054@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>