scsi: be2iscsi: Add checks to validate CID alloc/free

Set CID slot to 0xffff to indicate empty.
Check if connection already exists in conn_table before binding.
Check if endpoint already NULL before putting back CID.
Break ep->conn link in free_ep to ignore completions after freeing.

Signed-off-by: Jitendra Bhivare <jitendra.bhivare@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>