When CONFIG_FRAME_POINTER=y, calling dump_stack() can always trigger
NULL pointer dereference panic similar as below:

[    0.396060] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.13.0-rc5+ #47
[    0.396692] Hardware name: riscv-virtio,qemu (DT)
[    0.397176] Call Trace:
[    0.398191] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000960
[    0.399487] Oops [#1]
[    0.399739] Modules linked in:
[    0.400135] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.13.0-rc5+ #47
[    0.400570] Hardware name: riscv-virtio,qemu (DT)
[    0.400926] epc : walk_stackframe+0xc4/0xdc
[    0.401291]  ra : dump_backtrace+0x30/0x38
[    0.401630] epc : ffffffff80004922 ra : ffffffff8000496a sp : ffffffe000f3bd00
[    0.402115]  gp : ffffffff80cfdcb8 tp : ffffffe000f30000 t0 : ffffffff80d0b0cf
[    0.402602]  t1 : ffffffff80d0b0c0 t2 : 0000000000000000 s0 : ffffffe000f3bd60
[    0.403071]  s1 : ffffffff808bc2e8 a0 : 0000000000001000 a1 : 0000000000000000
[    0.403448]  a2 : ffffffff803d7088 a3 : ffffffff808bc2e8 a4 : 6131725dbc24d400
[    0.403820]  a5 : 0000000000001000 a6 : 0000000000000002 a7 : ffffffffffffffff
[    0.404226]  s2 : 0000000000000000 s3 : 0000000000000000 s4 : 0000000000000000
[    0.404634]  s5 : ffffffff803d7088 s6 : ffffffff808bc2e8 s7 : ffffffff80630650
[    0.405085]  s8 : ffffffff80912a80 s9 : 0000000000000008 s10: ffffffff804000fc
[    0.405388]  s11: 0000000000000000 t3 : 0000000000000043 t4 : ffffffffffffffff
[    0.405616]  t5 : 000000000000003d t6 : ffffffe000f3baa8
[    0.405793] status: 0000000000000100 badaddr: 0000000000000960 cause: 000000000000000d
[    0.406135] [<ffffffff80004922>] walk_stackframe+0xc4/0xdc
[    0.407032] [<ffffffff8000496a>] dump_backtrace+0x30/0x38
[    0.407797] [<ffffffff803d7100>] show_stack+0x40/0x4c
[    0.408234] [<ffffffff803d9e5c>] dump_stack+0x90/0xb6
[    0.409019] [<ffffffff8040423e>] ptdump_init+0x20/0xc4
[    0.409681] [<ffffffff800015b6>] do_one_initcall+0x4c/0x226
[    0.410110] [<ffffffff80401094>] kernel_init_freeable+0x1f4/0x258
[    0.410562] [<ffffffff803dba88>] kernel_init+0x22/0x148
[    0.410959] [<ffffffff800029e2>] ret_from_exception+0x0/0x14
[    0.412241] ---[ end trace b2ab92c901b96251 ]---
[    0.413099] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

The reason is the task is NULL when we finally call walk_stackframe()
the NULL is passed from __dump_stack():

|static void __dump_stack(void)
|{
|        dump_stack_print_info(KERN_DEFAULT);
|        show_stack(NULL, NULL, KERN_DEFAULT);
|}

Fix this issue by checking "task == NULL" case in walk_stackframe().

Fixes: eac2f305 ("riscv: stacktrace: fix the riscv stacktrace when CONFIG_FRAME_POINTER enabled")
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Wende Tan <twd2.me@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>