[SCSI] libiscsi: fix logout pdu processing

According to the iscsi RFC, we cannot send other requests if
we have sent a logout pdu. This patch enforces this requirement
by blocking the session and suspending the send thread. Userspace
decides if we restart the connection or if we just free everything.

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