iSCSI: NOPs should not have a CmdSN

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

Normally on an active connection, when a command response is received, the
initiator can acknowledge the response by setting ExpStatSN in the next
command sent.

However, on a relative inactive connection, it may be some time before the
next command is sent.  But the initiator should acknowledge the command
response in a timely fashion so the target can de-allocate the "replay"
resources for the command.  The initiator does this by sending NOP.

However, the NOP requires a CmdSN.  CmdSN is managed on a session wide basis.
StatSN is managed on a connection basis.  In the case where there are multiple
connections active in the session on different iSCSI NICs, the session manager
that resides in the host manages the CmdSN.  Therefore, if the connection
manager on an iSCSI NIC wants to send a NOP to acknowledge StatSN on an idle
connection, it must ask the session manager to do so, since it doesn't have
visibility to the next CmdSN.

To resolve this complexity, I propose removing CmdSN from the NOP.  I see no
reason why NOPs need to be processed "in order" across the session.

-Matt Wakeley
Agilent Technologies