block: Mark bdrv_get_cumulative_perm() and callers GRAPH_RDLOCK

The function reads the parents list, so it needs to hold the graph lock.

This happens to result in BlockDriver.bdrv_set_perm() to be called with
the graph lock held. For consistency, make it the same for all of the
BlockDriver callbacks for updating permissions and annotate the function
pointers with GRAPH_RDLOCK_PTR.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230911094620.45040-15-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>