scsi: target/iscsi: Make sure PDU processing continues if parsing a command fails

Currently the iSCSI target driver sends a CHECK CONDITION code back to the
initiator if the immediate data buffer is too large but it does not discard
that immediate data buffer. The result is that the iSCSI target driver
attempts to parse the immediate data itself as iSCSI PDUs and that all
further iSCSI communication fails. Fix this by receiving and discarding too
large immediate data buffers.

Cc: Mike Christie <mchristi@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>