block: fail op_is_write() requests to read-only partitions

block: fail op_is_write() requests to read-only partitions

Regular block device writes go through blkdev_write_iter(), which does
bdev_read_only(), while zeroout/discard/etc requests are never checked,
both userspace- and kernel-triggered.  Add a generic catch-all check to
generic_make_request_checks() to actually enforce ioctl(BLKROSET) and
set_disk_ro(), which is used by quite a few drivers for things like
snapshots, read-only backing files/images, etc.

Reviewed-by: Sagi Grimberg <[email protected]>
Signed-off-by: Ilya Dryomov <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>