block/swim: Fix IO error at end of medium

Reading to the end of a 720K disk results in an IO error instead of EOF
because the block layer thinks the disk has 2880 sectors. (Partly this
is a result of inverted logic of the ONEMEG_MEDIA bit that's now fixed.)

Initialize the density and head count in swim_add_floppy() to agree
with the device size passed to set_capacity() during drive probe.

Call set_capacity() again upon device open, after refreshing the density
and head count values.

Cc: Laurent Vivier <lvivier@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: stable@vger.kernel.org # v4.14+
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Acked-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>