powerpc/ptrace: Enable in transaction NT_PRFPREG ptrace requests

This patch enables in transaction NT_PRFPREG ptrace requests.
The function fpr_get which gets the running value of all FPR
registers and the function fpr_set which sets the running
value of of all FPR registers work on the running set of FPR
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>