selftests/bpf: Test sockmap update when socket has ULP

Cover the scenario when we cannot insert a socket into the sockmap, because
it has it is using ULP. Failed insert should not have any effect on the ULP
state. This is a regression test.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/r/20220623091231.417138-1-jakub@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>