This adds GRAPH_RDLOCK annotations to declare that callers of
bdrv_probe_blocksizes() need to hold a reader lock for the graph because
it calls bdrv_filter_bs(), which accesses bs->file/backing.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231027155333.420094-2-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>