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> # 3.7.x-
Fixes: 478fcb2c ("arm64: Debugging support")
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Acked-by: NWill Deacon <Will.Deacon@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>