Add hweight64()

Signed-off-by: Jens Axboe <axboe@kernel.dk>
windows: use hweight64(), it's a 64-bit type

Courtesy of Bruce.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Use unsigned long long for the uint64_t type

Signed-off-by: Jens Axboe <axboe@kernel.dk>