riscv: Use READ_ONCE_NOCHECK in imprecise unwinding stack mode

In the Linux kernel, the following vulnerability has been resolved:

riscv: Use READ_ONCE_NOCHECK in imprecise unwinding stack mode

When CONFIG_FRAME_POINTER is unset, the stack unwinding function
walk_stackframe randomly reads the stack and then, when KASAN is enabled,
it can lead to the following backtrace:

[ 0.000000] ==================================================================
[ 0.000000] BUG: KASAN: stack-out-of-bounds in walk_stackframe+0xa6/0x11a
[ 0.000000] Read of size 8 at addr ffffffff81807c40 by task swapper/0
[ 0.000000]
[ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.2.0-12919-g24203e6db61f #43
[ 0.000000] Hardware name: riscv-virtio,qemu (DT)
[ 0.000000] Call Trace:
[ 0.000000] [<ffffffff80007ba8>] walk_stackframe+0x0/0x11a
[ 0.000000] [<ffffffff80099ecc>] init_param_lock+0x26/0x2a
[ 0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[ 0.000000] [<ffffffff80c49c80>] dump_stack_lvl+0x22/0x36
[ 0.000000] [<ffffffff80c3783e>] print_report+0x198/0x4a8
[ 0.000000] [<ffffffff80099ecc>] init_param_lock+0x26/0x2a
[ 0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[ 0.000000] [<ffffffff8015f68a>] kasan_report+0x9a/0xc8
[ 0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[ 0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[ 0.000000] [<ffffffff8006e99c>] desc_make_final+0x80/0x84
[ 0.000000] [<ffffffff8009a04e>] stack_trace_save+0x88/0xa6
[ 0.000000] [<ffffffff80099fc2>] filter_irq_stacks+0x72/0x76
[ 0.000000] [<ffffffff8006b95e>] devkmsg_read+0x32a/0x32e
[ 0.000000] [<ffffffff8015ec16>] kasan_save_stack+0x28/0x52
[ 0.000000] [<ffffffff8006e998>] desc_make_final+0x7c/0x84
[ 0.000000] [<ffffffff8009a04a>] stack_trace_save+0x84/0xa6
[ 0.000000] [<ffffffff8015ec52>] kasan_set_track+0x12/0x20
[ 0.000000] [<ffffffff8015f22e>] __kasan_slab_alloc+0x58/0x5e
[ 0.000000] [<ffffffff8015e7ea>] __kmem_cache_create+0x21e/0x39a
[ 0.000000] [<ffffffff80e133ac>] create_boot_cache+0x70/0x9c
[ 0.000000] [<ffffffff80e17ab2>] kmem_cache_init+0x6c/0x11e
[ 0.000000] [<ffffffff80e00fd6>] mm_init+0xd8/0xfe
[ 0.000000] [<ffffffff80e011d8>] start_kernel+0x190/0x3ca
[ 0.000000]
[ 0.000000] The buggy address belongs to stack of task swapper/0
[ 0.000000] and is located at offset 0 in frame:
[ 0.000000] stack_trace_save+0x0/0xa6
[ 0.000000]
[ 0.000000] This frame has 1 object:
[ 0.000000] [32, 56) 'c'
[ 0.000000]
[ 0.000000] The buggy address belongs to the physical page:
[ 0.000000] page:(____ptrval____) refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x81a07
[ 0.000000] flags: 0x1000(reserved|zone=0)
[ 0.000000] raw: 0000000000001000 ff600003f1e3d150 ff600003f1e3d150 0000000000000000
[ 0.000000] raw: 0000000000000000 0000000000000000 00000001ffffffff
[ 0.000000] page dumped because: kasan: bad access detected
[ 0.000000]
[ 0.000000] Memory state around the buggy address:
[ 0.000000] ffffffff81807b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 0.000000] ffffffff81807b80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 0.000000] >ffffffff81807c00: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 00 f3
[ 0.000000] ^
[ 0.000000] ffffffff81807c80: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
[ 0.000000] ffffffff81807d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 0.000000] ==================================================================

Fix that by using READ_ONCE_NOCHECK when reading the stack in imprecise
mode.