powerpc/ptrace: Preserve previous TM fprs/vsrs on short regset write

powerpc/ptrace: Preserve previous TM fprs/vsrs on short regset write

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

Fixes: e388ee910cfb ("powerpc/ptrace: Enable support for NT_PPC_CVSX")
Fixes: 7e6cf5700225 ("powerpc/ptrace: Enable support for NT_PPC_CFPR")
Cc: stable@vger.kernel.org # v4.8+
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>