[SCSI] scsi_transport_iscsi: Add conn login, kernel to user, event to support offload session login.

Offload drivers like qla4xxx will offload the sending of the login/logout
pdus still, so this patch adds iscsi_conn_login_event which is
used by these types of drivers to notify userspace that the connection
has changed state.

It also adds a iscsi_is_session_online helper so the lld
can query the sessions state field.
Signed-off-by: Manish Rangankar <manish.rangankar@qlogic.com>
Signed-off-by: Lalit Chandivade <lalit.chandivade@qlogic.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>