iSCSI: SNACK wording clarification

["Matt Wakeley" <matt_wakeley@agilent.com>]

The word SNACK should be defined (Selective Negative ACKnowledgement)

The discussion of SNACK indicates that it is used for the "retransmission of
status, data or R2T PDUs".  The use of the word "status" can be confused to
imply that only SCSI command status can be retransmitted.  Instead, the word
"response" should be used since SNACK is used to retransmit any PDU that
advances StatSN (such as task management responses, text responses, NOP-IN).

-Matt