parisc: Ensure userspace privilege for ptraced processes in regset functions

On parisc the privilege level of a process is stored in the lowest two bits of
the instruction pointers (IAOQ0 and IAOQ1). On Linux we use privilege level 0
for the kernel and privilege level 3 for user-space. So userspace should not be
allowed to modify IAOQ0 or IAOQ1 of a ptraced process to change it's privilege
level to e.g. 0 to try to gain kernel privileges.

This patch prevents such modifications in the regset support functions by
always setting the two lowest bits to one (which relates to privilege level 3
for user-space) if IAOQ0 or IAOQ1 are modified via ptrace regset calls.

Link: https://bugs.gentoo.org/481768
Cc: <stable@vger.kernel.org> # v4.7+
Tested-by: Rolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: Helge Deller <deller@gmx.de>