mpt-scsi: try to enumerate luns with REPORT LUNS

mpt-scsi: try to enumerate luns with REPORT LUNS

Enumerate active luns with REPORT LUNS and, if that fails[*], fall back
to sequentially enumerating them up to (arbitrarily chosen) #8.

Note that this patch also makes mpt_scsi_cmd accept luns other than 0;
I've no idea what was the original motivation not to, and what can break
due to this change (nothing broke in my basic tests with QEMU).

[*] in current QEMU, REPORT LUNS does fail in mptsas1068 because the
returned data is smaller than the allocation length which is (wrongly)
considered an underflow.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>