block: move GENHD_FL_BLOCK_EVENTS_ON_EXCL_WRITE to disk->event_flags

GENHD_FL_BLOCK_EVENTS_ON_EXCL_WRITE is all about the event reporting
mechanism, so move it to the event_flags field.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211122130625.1136848-3-hch@lst.de


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