zram: replace fsync_bdev with sync_blockdev

zram: replace fsync_bdev with sync_blockdev

When calling fsync_bdev(), zram driver guarantees that the bdev won't be
opened by anyone, then there can't be one active fs/superblock over the
zram bdev, so replace fsync_bdev with sync_blockdev.

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Link: https://lore.kernel.org/r/20211025025426.2815424-5-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>