arm64: mte: introduce a per-CPU tag checking mode preference

arm64: mte: introduce a per-CPU tag checking mode preference

Add a per-CPU sysfs node, mte_tcf_preferred, that allows the preferred
tag checking mode to be configured. The current possible values are
async and sync.

Link: https://linux-review.googlesource.com/id/I7493dcd533a2785a1437b16c3f6b50919f840854
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210727205300.2554659-5-pcc@google.com
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>