[SCSI] libiscsi: flush work before freeing connection

It's possible that we call iscsi_xmitworker after iscsi_conn_release
which causes a oops. This patch flushes the workqueue.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>