lightnvm: pblk: fix read_bitmap for 32bit archs

If using pblk on a 32bit architecture, and there is a need to
perform a partial read, the partial read bitmap will only have
allocated 32 entries, where as 64 are needed.

Make sure that the read_bitmap is initialized to 64bits on 32bit
architectures as well.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Reviewed-by: Igor Konopko <igor.j.konopko@intel.com>
Reviewed-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>