Received-message CRCs (hdr_crc, aux_crc) are never validated

Summary

The transmit side computes aux_crc and hdr_crc but the receive path never verifies either field for incoming messages — the CRC fields are stored and ignored. An active on-path attacker (MITM on the TCP link between cluster nodes) can tamper with message bodies undetected; combined with DF-5825-class state-machine weaknesses this removes a defense-in-depth layer the protocol documents as required integrity (dmsg.h:187-196). Impact limited to integrity of an already trust-free channel (the peer itself gains nothing). Verify-iscsi_crc32-and-drop-link fix proposed.