powerpc/tm: Fix more userspace r13 corruption

In the Linux kernel, the following vulnerability has been resolved:

powerpc/tm: Fix more userspace r13 corruption

Commit cf13435b730a ("powerpc/tm: Fix userspace r13 corruption") fixes a
problem in treclaim where a SLB miss can occur on the
thread_struct->ckpt_regs while SCRATCH0 is live with the saved user r13
value, clobbering it with the kernel r13 and ultimately resulting in
kernel r13 being stored in ckpt_regs.

There is an equivalent problem in trechkpt where the user r13 value is
loaded into r13 from chkpt_regs to be recheckpointed, but a SLB miss
could occur on ckpt_regs accesses after that, which will result in r13
being clobbered with a kernel value and that will get recheckpointed and
then restored to user registers.

The same memory page is accessed right before this critical window where
a SLB miss could cause corruption, so hitting the bug requires the SLB
entry be removed within a small window of instructions, which is
possible if a SLB related MCE hits there. PAPR also permits the
hypervisor to discard this SLB entry (because slb_shadow->persistent is
only set to SLB_NUM_BOLTED) although it's not known whether any
implementations would do this (KVM does not). So this is an extremely
unlikely bug, only found by inspection.

Fix this by also storing user r13 in a temporary location on the kernel
stack and don't change the r13 register from kernel r13 until the RI=0
critical section that does not fault.

The SCRATCH0 change is not strictly part of the fix, it's only used in
the RI=0 section so it does not have the same problem as the previous
SCRATCH0 bug.