powerpc/kprobes: Fix Oops by passing ppc_inst as a pointer to emulate_step() on ppc32

Trying to use a kprobe on ppc32 results in the below splat:
    BUG: Unable to handle kernel data access on read at 0x7c0802a6
    Faulting instruction address: 0xc002e9f0
    Oops: Kernel access of bad area, sig: 11 [#1]
    BE PAGE_SIZE=4K PowerPC 44x Platform
    Modules linked in:
    CPU: 0 PID: 89 Comm: sh Not tainted 5.13.0-rc1-01824-g3a81c0495fdb #7
    NIP:  c002e9f0 LR: c0011858 CTR: 00008a47
    REGS: c292fd50 TRAP: 0300   Not tainted  (5.13.0-rc1-01824-g3a81c0495fdb)
    MSR:  00009000 <EE,ME>  CR: 24002002  XER: 20000000
    DEAR: 7c0802a6 ESR: 00000000
    <snip>
    NIP [c002e9f0] emulate_step+0x28/0x324
    LR [c0011858] optinsn_slot+0x128/0x10000
    Call Trace:
     opt_pre_handler+0x7c/0xb4 (unreliable)
     optinsn_slot+0x128/0x10000
     ret_from_syscall+0x0/0x28

The offending instruction is:
    81 24 00 00     lwz     r9,0(r4)

Here, we are trying to load the second argument to emulate_step():
struct ppc_inst, which is the instruction to be emulated. On ppc64,
structures are passed in registers when passed by value. However, per
the ppc32 ABI, structures are always passed to functions as pointers.
This isn't being adhered to when setting up the call to emulate_step()
in the optprobe trampoline. Fix the same.

Fixes: eacf4c02

 ("powerpc: Enable OPTPROBES on PPC32")
Cc: stable@vger.kernel.org
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5bdc8cbc9a95d0779e27c9ddbf42b40f51f883c0.1624425798.git.christophe.leroy@csgroup.eu