s390/ptrace: fix guarded storage regset handling

If the guarded storage regset for current is supposed to be changed,
the regset from user space is copied directly into the guarded storage
control block.

If then the process gets scheduled away while the control block is
being copied and before the new control block has been loaded, the
result is random: the process can be scheduled away due to a page
fault or preemption. If that happens the already copied parts will be
overwritten by save_gs_cb(), called from switch_to().

Avoid this by copying the data to a temporary buffer on the stack and
do the actual update with preemption disabled.

Fixes: f5bbd721 ("s390/ptrace: guarded storage regset for the current task")
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>