powerpc: Fix audit crash due to save/restore PPR changes

The current mainline crashes when hitting userspace with the following:

kernel BUG at kernel/auditsc.c:1769!
cpu 0x1: Vector: 700 (Program Check) at [c000000023883a60]
    pc: c0000000001047a8: .__audit_syscall_entry+0x38/0x130
    lr: c00000000000ed64: .do_syscall_trace_enter+0xc4/0x270
    sp: c000000023883ce0
   msr: 8000000000029032
  current = 0xc000000023800000
  paca    = 0xc00000000f080380   softe: 0        irq_happened: 0x01
    pid   = 1629, comm = start_udev
kernel BUG at kernel/auditsc.c:1769!
enter ? for help
[c000000023883d80] c00000000000ed64 .do_syscall_trace_enter+0xc4/0x270
[c000000023883e30] c000000000009b08 syscall_dotrace+0xc/0x38
 --- Exception: c00 (System Call) at 0000008010ec50dc

Bisecting found the following patch caused it:

commit 44e9309f1f357794b7ae93d5f3e3e6f11d2b8a7f
Author: Haren Myneni <haren@linux.vnet.ibm.com>
powerpc: Implement PPR save/restore

It was found this patch corrupted r9 when calling
SET_DEFAULT_THREAD_PPR()

Using r10 as a scratch register instead of r9 solved the problem.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>