block: Drain nodes before inactivating them

So far the assumption has always been that if we try to inactivate a
node, it is already idle. This doesn't hold true any more if we allow
inactivating exported nodes because we can't know when new external
requests come in.

Drain the node around setting BDRV_O_INACTIVE so that requests can't
start operating on an active node and then in the middle it suddenly
becomes inactive. With this change, it's enough for exports to check
for new requests that they operate on an active node (or, like reads,
are allowed even on an inactive node).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250204211407.381505-12-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -7032,7 +7032,9 @@ bdrv_inactivate_recurse(BlockDriverState *bs, bool top_level)
         return -EPERM;
     }
 
+    bdrv_drained_begin(bs);
     bs->open_flags |= BDRV_O_INACTIVE;
+    bdrv_drained_end(bs);
 
     /*
      * Update permissions, they may differ for inactive nodes.