block: don't warn for flush on read-only device

Don't warn for a flush issued to a read-only device. It's not strictly
a writable command, as it doesn't change any on-media data by itself.

Reported-by: Stefan Agner <stefan@agner.ch>
Fixes: 721c7fc7 ("block: fail op_is_write() requests to read-only partitions")
Signed-off-by: Jens Axboe <axboe@kernel.dk>