Add architecture specific implementation details for KFENCE and enable
KFENCE on ARM. In particular, this implements the required interface in
 <asm/kfence.h>.

KFENCE requires that attributes for pages from its memory pool can
individually be set. Therefore, force the kfence pool to be mapped
at page granularity.

Testing this patch using the testcases in kfence_test.c and all passed
with or without ARM_LPAE.
Acked-by: NMarco Elver <elver@google.com>
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NRussell King (Oracle) <rmk+kernel@armlinux.org.uk>