bpf: sockmap, test for unconnected af_unix sock

Add test to sockmap_basic to ensure af_unix sockets that are not connected
can not be added to the map. Ensure we keep DGRAM sockets working however
as these will not be connected typically.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/r/20231201180139.328529-3-john.fastabend@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>