s390/kernel: fix ptrace peek/poke for floating point registers

s390/kernel: fix ptrace peek/poke for floating point registers

commit 55a423b6f105fa323168f15f4bb67f23b21da44e upstream.

git commit 155e839a814834a3b4b31e729f4716e59d3d2dd4
"s390/kernel: dynamically allocate FP register save area"
introduced a regression in regard to ptrace.

If the vector register extension is not present or unused the
ptrace peek of a floating pointer register return incorrect data
and the ptrace poke to a floating pointer register overwrites the
task structure starting at task->thread.fpu.fprs.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>