ARM: 9476/1: mm: fix kexec and hibernation with CONFIG_CPU_TTBR0_PAN

ARM: 9476/1: mm: fix kexec and hibernation with CONFIG_CPU_TTBR0_PAN

Commit 7af5b901e847 ("ARM: 9358/2: Implement PAN for LPAE by TTBR0
page table walks disablement") implemented PAN for LPAE kernels by
setting TTBCR.EPD0 on every kernel entry, disabling TTBR0 page-table
walks while running in kernel mode. The commit correctly updated
cpu_suspend() in arch/arm/kernel/suspend.c, but missed two other code
paths that switch the CPU to the identity mapping before jumping to
low-PA (TTBR0-range) physical addresses:

1. setup_mm_for_reboot() in arch/arm/mm/idmap.c, used by the kexec
   reboot path. With TTBCR.EPD0 still set, the subsequent branch to
   the identity-mapped cpu_v7_reset causes a PrefetchAbort because the
   TTBR0 page-table walk needed to resolve the identity-mapped address
   is disabled. This manifests as a hard hang or "bad PC value" panic
   on LPAE kernels booted on CPUs that strictly enforce EPD0 for
   instruction fetch (e.g. Cortex-A53 in AArch32 mode) while the same
   image may accidentally work on Cortex-A15 due to microarchitectural
   differences in EPD0 enforcement.

2. arch_restore_image() in arch/arm/kernel/hibernate.c, which calls
   cpu_switch_mm(idmap_pgd, &init_mm) directly without going through
   setup_mm_for_reboot(), leaving TTBCR.EPD0 set while the identity
   mapping is active.

Fix both sites by calling uaccess_save_and_enable() before switching
to the identity mapping, mirroring what the original commit did for
cpu_suspend().

Assisted-by: Cursor:claude-sonnet-4.6
Fixes: 7af5b901e847 ("ARM: 9358/2: Implement PAN for LPAE by TTBR0 page table walks disablement")
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>