ARM: 8643/3: arm/ptrace: Preserve previous registers for short regset write

commit 228dbbfb upstream.

Ensure that if userspace supplies insufficient data to
PTRACE_SETREGSET to fill all the registers, the thread's old
registers are preserved.

Fixes: 5be6f62b ("ARM: 6883/1: ptrace: Migrate to regsets framework")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>