MFC r272401, r272402: Rework the logic of sequential SCSI LUN scanner.

MFC r272401, r272402: Rework the logic of sequential SCSI LUN scanner.

Previous logic was not differentiating disconnected LUNs and absent targets.
That made it to stop scan if LUN 0 was not found for any reason.  That made
problematic, for example, using iSCSI targets declaring SPC-2 compliance and
having no LUN 0 configured.

The new logic continues sequential LUN scan if:
 -- we have more configured LUNs that need recheck;
 -- this LUN is connected and its SCSI version allows more LUNs;
 -- this LUN is disconnected, its SCSI version allows more LUNs and we
    guess they may be connected (we haven't scanned first 8 LUNs yet or
    kern.cam.cam_srch_hi sysctl is set to scan more).

git-svn-id: svn://svn.freebsd.org/base/stable/10@273974 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f