selftests/bpf: check epoll readiness during reuseport migration

selftests/bpf: check epoll readiness during reuseport migration

Inside migrate_dance(), add epoll checks around shutdown() to
verify that the target listener is not ready before shutdown()
and becomes ready immediately after shutdown() triggers migration.

Cover TCP_ESTABLISHED and TCP_SYN_RECV. Exclude TCP_NEW_SYN_RECV
as it depends on later handshake completion.

Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Zhenzhong Wu <jt26wzz@gmail.com>
Link: https://patch.msgid.link/20260422024554.130346-3-jt26wzz@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>