block: mark bdrv_open_child_common() and its callers GRAPH_UNLOCKED

block: mark bdrv_open_child_common() and its callers GRAPH_UNLOCKED

The function bdrv_open_child_common() calls
bdrv_graph_wrlock_drained(), which must be called with the graph
unlocked. Mark it and its two callers bdrv_open_file_child() and
bdrv_open_child() as GRAPH_UNLOCKED. This requires temporarily
unlocking in vmdk_parse_extents() and making the locked section
shorter in vmdk_open().

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-48-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>