powerpc/tm: Drop tm_orig_msr from thread_struct

Currently tm_orig_msr is getting used during process context switch only.
Then there is ckpt_regs which saves the checkpointed userspace context
The MSR slot contained in ckpt_regs structure can be used during process
context switch instead of tm_orig_msr, thus allowing us to drop it from
thread_struct structure. This patch does that change.

Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>