block: add a new set_read_only method

[ Upstream commit e00adcad ]

Add a new method to allow for driver-specific processing when setting or
clearing the block device read-only state.  This allows to replace the
cumbersome and error-prone override of the whole ioctl implementation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: 9674f54e ("md: Don't clear MD_CLOSING when the raid is about to stop")
Signed-off-by: Sasha Levin <sashal@kernel.org>