Implement 'flat space LUN addressing', which allows us to raise the max_lun
limitation to 16384. The maximum number of LUNs prior to this patch was
256.

Link: https://lore.kernel.org/r/20200821042249.5097-1-dgilbert@interlog.comSuggested-by: NHannes Reinecke <hare@suse.de>
Signed-off-by: NDouglas Gilbert <dgilbert@interlog.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>