md/raid5-cache: fix payload endianness problem in raid5-cache

The payload->header.type and payload->size are little-endian, so just
convert them to the right byte order.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Cc: <stable@vger.kernel.org> #v4.10+
Signed-off-by: Shaohua Li <shli@fb.com>