rbd: decouple header read-in from updating rbd_dev->header

commit 510a7330c82a7754d5df0117a8589e8a539067c7 upstream.

Make rbd_dev_header_info() populate a passed struct rbd_image_header
instead of rbd_dev->header and introduce rbd_dev_update_header() for
updating mutable fields in rbd_dev->header upon refresh.  The initial
read-in of both mutable and immutable fields in rbd_dev_image_probe()
passes in rbd_dev->header so no update step is required there.

rbd_init_layout() is now called directly from rbd_dev_image_probe()
instead of individually in format 1 and format 2 implementations.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
[idryomov@gmail.com: backport to 5.4: _rbd_dev_v2_snap_features()
 doesn't have read_only param]
Signed-off-by: Sasha Levin <sashal@kernel.org>