ARM: 9475/1: entry: use byte load for KASAN VMAP stack shadow

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

ARM: 9475/1: entry: use byte load for KASAN VMAP stack shadow

Commit 44e9a3bb76e5 ("ARM: 9430/1: entry: Do a dummy read from

VMAP shadow") added a dummy read from the KASAN VMAP stack shadow in

__switch_to(). The read uses ldr, but the KASAN shadow address is

byte-granular and is not guaranteed to be word aligned.

ARMv5 faults unaligned word loads. With CONFIG_KASAN_VMALLOC and

CONFIG_VMAP_STACK enabled, ARM926/VersatilePB crashes in __switch_to()

with an alignment exception before reaching init.

Use ldrb for the dummy shadow access. The code only needs to fault in the

shadow mapping if the stack shadow is missing, so a byte load is sufficient

and matches the granularity of KASAN shadow memory.