powerpc: Fix MAX_STACK_TRACE_ENTRIES too low warning !

This patch tries to fix the following BUG report:

[    0.012313] BUG: MAX_STACK_TRACE_ENTRIES too low!
[    0.012318] turning off the locking correctness validator.
[    0.012321] Call Trace:
[    0.012330] [c00000017666f6d0] [c000000000012128] .show_stack+0x78/0x184 (unreliable)
[    0.012339] [c00000017666f780] [c0000000000b6348] .save_trace+0x12c/0x14c
[    0.012345] [c00000017666f800] [c0000000000b7448] .mark_lock+0x2bc/0x710
[    0.012351] [c00000017666f8b0] [c0000000000bb198] .__lock_acquire+0x748/0xaec
[    0.012357] [c00000017666f9b0] [c0000000000bb684] .lock_acquire+0x148/0x194
[    0.012365] [c00000017666fa80] [c00000000069371c] .mutex_lock_nested+0x84/0x4ec
[    0.012372] [c00000017666fb90] [c000000000096998] .smpboot_register_percpu_thread+0x3c/0x10c
[    0.012380] [c00000017666fc30] [c0000000009ba910] .spawn_ksoftirqd+0x28/0x48
[    0.012386] [c00000017666fcb0] [c00000000000a98c] .do_one_initcall+0xd8/0x1d0
[    0.012392] [c00000017666fd60] [c00000000000b1f8] .kernel_init+0x120/0x398

[    0.012398] [c00000017666fe30] [c000000000009ad4] .ret_from_kernel_thread+0x5c/0x64
[    0.012404] [c00000017666fa00] [c00000017666fb20] 0xc00000017666fb20
[    0.012410] [c00000017666fa80] [c00000000069371c] .mutex_lock_nested+0x84/0x4ec
[    0.012416] [c00000017666fb90] [c000000000096998] .smpboot_register_percpu_thread+0x3c/0x10c
[    0.012422] [c00000017666fc30] [c0000000009ba910] .spawn_ksoftirqd+0x28/0x48
[    0.012427] [c00000017666fcb0] [c00000000000a98c] .do_one_initcall+0xd8/0x1d0
[    0.012433] [c00000017666fd60] [c00000000000b1f8] .kernel_init+0x120/0x398

[    0.012439] [c00000017666fe30] [c000000000009ad4] .ret_from_kernel_thread+0x5c/0x64
.......

The reason is that the back chain of c00000017666fe30
(ret_from_kernel_thread) contains some invalid value, which might form a
loop.
Signed-off-by: NLi Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/entry_64.S

@@ -374,6 +374,8 @@ _GLOBAL(ret_from_kernel_thread)
 	bl	.schedule_tail
 	REST_NVGPRS(r1)
 	REST_GPR(2,r1)
+	li	r3,0
+	std	r3,0(r1)
 	mtlr	r14
 	mr	r3,r15
 	blrl