arm64: csum: cast to the proper type

The last line of ip_fast_csum() calls csum_fold(), forcing the
type of the argument passed to be u32.  But csum_fold() takes a
__wsum argument (which is __u32 __bitwise for arm64).  As long
as we're forcing the cast, cast it to the right type.

Signed-off-by: Alex Elder <elder@linaro.org>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20210315012650.1221328-1-elder@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>