[SCSI] scsi_transport_fc: Introduce disable_target_scan flag

This change has already been discussed on linux-scsi:
http://marc.info/?t=118771096400003
http://marc.info/?t=118760913100005



Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Acked-by: James Smart <James.Smart@Emulex.Com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>