swim: add a floppy registration bool which triggers del_gendisk()

Instead of calling del_gendisk() on exit alone, let's add
a registration bool to the floppy disk state, this way this can
be done on the shared caller, swim_cleanup_floppy_disk().

This will be more useful in subsequent patches. Right now, this
just shuffles functionality out to a helper in a safe way.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20210927220302.1073499-10-mcgrof@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>