swim: fix a missing FMODE_ -> BLK_OPEN_ conversion in floppy_open

Fix a missing conversion to the new BLK_OPEN constant in swim.

Fixes: 05bdb996 ("block: replace fmode_t with a block-specific type for block open flags")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230620043051.707196-1-hch@lst.de


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