powerpc/pseries: Fix endianness while restoring of r3 in MCE handler.

During Machine Check interrupt on pseries platform, register r3 points
RTAS extended event log passed by hypervisor. Since hypervisor uses r3
to pass pointer to rtas log, it stores the original r3 value at the
start of the memory (first 8 bytes) pointed by r3. Since hypervisor
stores this info and rtas log is in BE format, linux should make
sure to restore r3 value in correct endian format.

Without this patch when MCE handler, after recovery, returns to code that
that caused the MCE may end up with Data SLB access interrupt for invalid
address followed by kernel panic or hang.

  Severe Machine check interrupt [Recovered]
    NIP [d00000000ca301b8]: init_module+0x1b8/0x338 [bork_kernel]
    Initiator: CPU
    Error type: SLB [Multihit]
      Effective address: d00000000ca70000
  cpu 0xa: Vector: 380 (Data SLB Access) at [c0000000fc7775b0]
      pc: c0000000009694c0: vsnprintf+0x80/0x480
      lr: c0000000009698e0: vscnprintf+0x20/0x60
      sp: c0000000fc777830
     msr: 8000000002009033
     dar: a803a30c000000d0
    current = 0xc00000000bc9ef00
    paca    = 0xc00000001eca5c00	 softe: 3	 irq_happened: 0x01
      pid   = 8860, comm = insmod
  vscnprintf+0x20/0x60
  vprintk_emit+0xb4/0x4b0
  vprintk_func+0x5c/0xd0
  printk+0x38/0x4c
  init_module+0x1c0/0x338 [bork_kernel]
  do_one_initcall+0x54/0x230
  do_init_module+0x8c/0x248
  load_module+0x12b8/0x15b0
  sys_finit_module+0xa8/0x110
  system_call+0x58/0x6c
  --- Exception: c00 (System Call) at 00007fff8bda0644
  SP (7fffdfbfe980) is in userspace

This patch fixes this issue.

Fixes: a08a53e ("powerpc/le: Enable RTAS events support")
Cc: stable@vger.kernel.org # v3.15+
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/pseries/ras.c

@@ -367,7 +367,7 @@ static struct rtas_error_log *fwnmi_get_errinfo(struct pt_regs *regs)
 	}
 
 	savep = __va(regs->gpr[3]);
-	regs->gpr[3] = savep[0];	/* restore original r3 */
+	regs->gpr[3] = be64_to_cpu(savep[0]);	/* restore original r3 */
 
 	h = (struct rtas_error_log *)&savep[1];
 	/* Use the per cpu buffer from paca to store rtas error log */