swim: simplify media change handling

floppy_revalidate mostly duplicates work already done in floppy_open
despite only beeing called from floppy_open.  Remove the function and
just clear the ->ejected flag directly under the right condition.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>