IB/iser: Signal iSCSI layer that transport is broken in error completions

Previously we notified iscsi layer about the connection layer when
we consumed all of our flush errors. This was racy as there
was no guarentee that iscsi_conn wasn't terminated by then (which ends
up in an invalid memory access). In case we got a non FLUSH error
completion, we are guarenteed that iscsi_conn is still alive. We should
notify iSCSI layer with iscsi_conn_failure to initiate error handling.

While we are at it, add a nice kernel-doc style documentation.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Ariel Nahum <arieln@mellanox.com>
Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>