arm64/ptrace: Preserve previous registers for short regset write

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

Cc: <stable@vger.kernel.org> # 4.3.x-
Fixes: 5d220ff9 ("arm64: Better native ptrace support for compat tasks")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <Will.Deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>