split hard read-only vs read-only policy v3

[Christoph Hellwig]

Hi Jens,

this series resurrects a patch from Martin to properly split the flag
indicating a disk has been set read-only by the hardware vs the userspace
policy set through the BLKROSET ioctl.

Note that the last patch only applies to for-next and not to
for-5.11/block.  I can hold it back for the first NVMe pull request after
Linus pulled the block tree.

A git tree is available here:

    git://git.infradead.org/users/hch/block.git block-hard-ro

Gitweb:

    http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/block-hard-ro

Changes since v3:
 - rebased to the latest block tree
 - indent commit log lines starting with a "#" to make sure git commit
   doesn't eat them

Changes since v2:
 - fix a few typos
 - add a patch to propagate the read-only status from the whole device to
   partitions
 - add a patch to remove a pointless check from bdev_read_only

Changes since v1:
 - don't propagate the policy flag from the whole disk to partitions
 - rebased on top of the merge block_device and hd_struct series