s390/cio: rename bitmap_size() -> idset_bitmap_size()

s390/cio: rename bitmap_size() -> idset_bitmap_size()

bitmap_size() is a pretty generic name and one may want to use it for
a generic bitmap API function. At the same time, its logic is not
"generic", i.e. it's not just `nbits -> size of bitmap in bytes`
converter as it would be expected from its name.
Add the prefix 'idset_' used throughout the file where the function
resides.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>