arm64: csum: Disable KASAN for do_csum()

do_csum() over-reads the source buffer and therefore abuses
READ_ONCE_NOCHECK() to avoid tripping up KASAN. In preparation for
READ_ONCE_NOCHECK() becoming a macro, and therefore losing its
'__no_sanitize_address' annotation, just annotate do_csum() explicitly
and fall back to normal loads.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>