sparse correctly warns that if mpa->private_data_size is __be16, then
doing += on it is wrong, even if we do += htons(<something>) -- on a
little endian system, carries will go the wrong way. Fix this up by
doing the addition in native byte order.
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>