btrfs_read_block_groups: Use enums to index

 btrfs_space_info->block_groups.

The current code uses integer literals to index
btrfs_space_info->block_groups[] array. Instead use corresponding
enums from 'enum btrfs_raid_types'.
Signed-off-by: Nchandan <chandan@linux.vnet.ibm.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/extent-tree.c

@@ -8441,9 +8441,13 @@ int btrfs_read_block_groups(struct btrfs_root *root)
 		 * avoid allocating from un-mirrored block group if there are
 		 * mirrored block groups.
 		 */
-		list_for_each_entry(cache, &space_info->block_groups[3], list)
+		list_for_each_entry(cache,
+				&space_info->block_groups[BTRFS_RAID_RAID0],
+				list)
 			set_block_group_ro(cache, 1);
-		list_for_each_entry(cache, &space_info->block_groups[4], list)
+		list_for_each_entry(cache,
+				&space_info->block_groups[BTRFS_RAID_SINGLE],
+				list)
 			set_block_group_ro(cache, 1);
 	}