ore: Support for partial component table

Users like the objlayout-driver would like to only pass
a partial device table that covers the IO in question.
For example exofs divides the file into raid-group-sized
chunks and only serves group_width number of devices at
a time.

The partiality is communicated by setting
ore_componets->first_dev and the array covers all logical
devices from oc->first_dev upto (oc->first_dev + oc->numdevs)

The ore_comp_dev() API receives a logical device index
and returns the actual present device in the table.
An out-of-range dev_index will BUG.

Logical device index is the theoretical device index as if
all the devices of a file are present. .i.e:
	total_devs = group_width * mirror_p1 * group_count
	0 <= dev_index < total_devs

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

fs/exofs/ore.c

@@ -62,6 +62,10 @@ static struct osd_obj_id *_ios_obj(struct ore_io_state *ios, unsigned index)
 
 static struct osd_dev *_ios_od(struct ore_io_state *ios, unsigned index)
 {
+	ORE_DBGMSG2("oc->first_dev=%d oc->numdevs=%d i=%d oc->ods=%p\n",
+		    ios->oc->first_dev, ios->oc->numdevs, index,
+		    ios->oc->ods);
+
 	return ore_comp_dev(ios->oc, index);
 }