block: use the holder as indication for exclusive opens

The current interface for exclusive opens is rather confusing as it
requires both the FMODE_EXCL flag and a holder.  Remove the need to pass
FMODE_EXCL and just key off the exclusive open off a non-NULL holder.

For blkdev_put this requires adding the holder argument, which provides
better debug checking that only the holder actually releases the hold,
but at the same time allows removing the now superfluous mode argument.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: David Sterba <dsterba@suse.com>		[btrfs]
Acked-by: Jack Wang <jinpu.wang@ionos.com>		[rnbd]
Link: https://lore.kernel.org/r/20230608110258.189493-16-hch@lst.de


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