stable-8.0: block/export: call blk_set_dev_ops and Fix null pointer dereference in error path

[Michael Tokarev]

Hi!

For some reason I picked up this commit:

commit de79b52604e43fdeba6cee4f5af600b62169f2d2
Author: Stefan Hajnoczi <stefanha@redhat.com>
Date:   Tue May 2 17:11:19 2023 -0400

    block/export: call blk_set_dev_ops(blk, NULL, NULL)

for stable-8.0.1.  However it turned out it fails iotests, up until
the fix, which is

commit a184563778f2b8970eb93291f08108e66432a575
Author: Kevin Wolf <kwolf@redhat.com>
Date:   Wed May 10 22:35:55 2023 +0200

    block/export: Fix null pointer dereference in error path

It's a good question why I picked the first one up to begin
with, but now, especially after not realizing iotests are not
run within CI framework automatically - I ended up with broken
v8.0.1 which I tagged earlier today.

What do you think, what is better for 8.0.1 - to revert first
commit or to add second on top?

Thanks!

/mjt