target: Minimize SCSI header #include directives

Only include SCSI initiator header files in target code that needs
these header files, namely the SCSI pass-through code and the tcm_loop
driver. Change SCSI_SENSE_BUFFERSIZE into TRANSPORT_SENSE_BUFFER in
target code because the former is intended for initiator code and the
latter for target code. With this patch the only initiator include
directives in target code that remain are as follows:

$ git grep -nHE 'include .scsi/(scsi.h|scsi_host.h|scsi_device.h|scsi_cmnd.h)' drivers/target drivers/infiniband/ulp/{isert,srpt} drivers/usb/gadget/legacy/tcm_*.[ch] drivers/{vhost,xen} include/{target,trace/events/target.h}
drivers/target/loopback/tcm_loop.c:29:#include <scsi/scsi.h>
drivers/target/loopback/tcm_loop.c:31:#include <scsi/scsi_host.h>
drivers/target/loopback/tcm_loop.c:32:#include <scsi/scsi_device.h>
drivers/target/loopback/tcm_loop.c:33:#include <scsi/scsi_cmnd.h>
drivers/target/target_core_pscsi.c:39:#include <scsi/scsi_device.h>
drivers/target/target_core_pscsi.c:40:#include <scsi/scsi_host.h>
drivers/xen/xen-scsiback.c:52:#include <scsi/scsi_host.h> /* SG_ALL */

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>