powerpc/ptrace: Enable in transaction NT_PPC_VMX ptrace requests

This patch enables in transaction NT_PPC_VMX ptrace requests. The
function vr_get which gets the running value of all VMX registers
and the function vr_set which sets the running value of of all VMX
registers work on the running set of VMX registers whose location
will be different if transaction is active. This patch makes these
functions adapt to situations when the transaction is active.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>