block: mark bdrv_reopen_queue() and bdrv_reopen_multiple() as GRAPH_UNLOCKED

block: mark bdrv_reopen_queue() and bdrv_reopen_multiple() as GRAPH_UNLOCKED

The function bdrv_reopen_queue() can call bdrv_drain_all_begin(),
which must be called with the graph unlocked.

The function bdrv_reopen_multiple() calls bdrv_reopen_prepare() which
must be called with the graph unlocked.

To mark bdrv_reopen_queue() as GRAPH_UNLOCKED, it is necessary to make
the locked section in reopen_backing_file() shorter.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-35-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>