ARM: 9350/1: fault: Implement copy_from_kernel_nofault_allowed()

Under PAN emulation when dumping backtraces from things like the
LKDTM EXEC_USERSPACE test[1], a double fault (which would hang a CPU)
would happen because of dump_instr() attempting to read a userspace
address. Make sure copy_from_kernel_nofault() does not attempt this
any more.

Closes: https://lava.sirena.org.uk/scheduler/job/497571
Link: https://lore.kernel.org/all/202401181125.D48DCB4C@keescook/

 [1]

Reported-by: Mark Brown <broonie@kernel.org>
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Cc: Wang Kefeng <wangkefeng.wang@huawei.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>