block: mark bdrv_parent_change_aio_context() GRAPH_RDLOCK

This is a small step in preparation to mark bdrv_drained_begin() as
GRAPH_UNLOCKED. More concretely, it allows marking the
change_aio_ctx() callback GRAPH_RDLOCK_PTR, which is the next step.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-6-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
1 file changed