nbd: Mark nbd_co_do_establish_connection() and callers GRAPH_RDLOCK

nbd: Mark nbd_co_do_establish_connection() and callers GRAPH_RDLOCK

This adds GRAPH_RDLOCK annotations to declare that callers of
nbd_co_do_establish_connection() need to hold a reader lock for the
graph.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230504115750.54437-11-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>