[SCSI] iscsi_tcp: add new conn error to indicate tcp conn closed

If a target closed the connection, we will detect it in the
state_changed or data_ready callout. This adds a new conn
error value to use for this problem, so it is not confused
with when the initiator throws a conn error and drops
the connection.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>