iSCSI: NOPs on discovery session

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

Julian,

I am not sure if this came up before, but we also need to include NOP-OUT to
the set of commands allowed on the discovery session. An initiator may keep
the discovery session active, and send NOP-OUT to ping the target, or in
response to a target ping.

-Ayman