True number of device openers in 2.5

[Andrey Borzenkov]

For single-partition devices it is just bd_openers but I am more intersted in 
multi-partition case. Here I need to know about sum of opens for all 
partitions including the whole device. It appears that for a container 
bd_openers is incremented for every open of itself and for the _first_ open 
of partition and bd_part_count is incremented for every (including first) 
open of partition. So bdev->bd_contains->bd_openers + 
bdev->bd_contains->bd_part_count really gives an access (number of opened 
partitions but this is unknown otherwise).

Thank you

-andrey