checklocks: enforce ICMP endpoint lock ownership

Packet delivery reads frozen under rcvMu, but freeze and thaw write it
under mu. Use rcvMu for both sides and annotate the receive state so
checklocks rejects this mismatch.

Guard ident with mu and require it in bindLocked. Carry the selected
identifier out of the synchronous network callbacks before assigning it
under mu, keeping the ownership visible to the analyzer.

Adapt the remaining coverage from #6642 to the current restore
lifecycle.

Assisted-by: Codex