block: fix the exclusive open mask in disk_scan_partitions

FMODE_EXEC has nothing to do with exclusive opens, and even is of
the wrong type.  We need to check for BLK_OPEN_EXCL here.

Fixes: 985958b8 ("block: fix wrong mode for blkdev_get_by_dev() from disk_scan_partitions()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230621124914.185992-1-hch@lst.de


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