usrsock: Do not return error when conn not found for an event

[wengzhe]

Summary

When rpmsg is in heavy load(e.g. rpmsgfs), it may result in out-of-ordered messages because it can trigger another callback in rpmsg_send. Then client side of usrsock may receive an event after socket is closed (e.g. SENDTO_READY event in UDP connect). Normally just drop these events might be acceptable, but we're returning error now, and triggers RPMSG_ASSERT outside.

Patches included:

• usrsock: Do not return error when conn not found for an event
  • If we receive an event to a conn after it's freed, log error and ignore it.
  • Also change log level to err when returning other negated errno, because if we return negated errno, we'll directly trigger RPMSG_ASSERT outside, so we need at least an error message to indicate the reason.
• usrsock: Add DebugAssert for poll setup result
  • We never check usrsock_rpmsg_poll_setup's result and assume it will success, so an assert should be better than retval.

Impact

• logic change: don't return error when usrsock conn not found for an event
• robustness: logs / debug asserts

Testing

CI & Vela

[xiaoxiang781216]

Let's ignore the ci broken which is fixed by #8611 and merge this simple change.