iSCSI-04: WN field

["Ayman Ghanem" <aghanem@cisco.com>]

Since the common case is likely to be a Basic Header Segment (BHS) followed
by either a data segment or no data, wouldn't be simpler to have bit-7 of
the WN field be defaulted to zero for the common case, and set it to 1 if we
have additional header segments?.

-Ayman