smb: fix SMB 3.1.1 signing for Kerberos sessions

[psycep]

Summary

SMB2/3 signing was broken for Kerberos-authenticated sessions against a
signing-required SMB 3.1.1 server (e.g. a modern Windows DC). The session
established, but every signed operation failed: outbound requests were
rejected with STATUS_ACCESS_DENIED ("permission denied") and inbound
response signatures failed to verify. NTLM against the same host worked
fine. This is a pre-existing bug, unrelated to the recent transport work.

Root cause

For SMB 3.1.1 the signing/encryption keys are derived with the preauth
integrity hash as the KDF context, so that hash must cover exactly the
right SESSION_SETUP messages (MS-SMB2 3.2.5.3): every request, every
interim response, and not the final STATUS_SUCCESS response.

The code hashed leg 1's request and response unconditionally, then hashed
"the final request" again during key derivation. For NTLM's two-leg
exchange that happens to land on the correct set:

neg + req1 + resp1 + req2

But Kerberos completes in a single leg (immediate STATUS_SUCCESS), so the
same code produced:

neg + req1 + resp1 + req1

— wrongly including the final success response and double-hashing the
request. Wrong hash → wrong signing key → outbound rejected, inbound
mismatched.

The Kerberos session-key handling itself (truncating the AES256 key to 16
bytes) is correct per MS-SMB2 and matches Impacket, and is unchanged.

Fix

• Fold each participating SESSION_SETUP message into
  PreauthIntegrityHashValue exactly once, at the point it is sent or
  received, excluding the final success response. Single-leg Kerberos now
  hashes neg + req1; two-leg NTLM is unchanged (neg + req1 + resp1 + req2).
• Re-enable inbound signature verification in verify(). It had been
  stubbed to log a mismatch warning and return true unconditionally — a
  debug bypass that disabled inbound signature checking entirely. The MAC
  is now compared in constant time.

Testing

Verified end-to-end against a Windows Server 2019 DC (SMB 3.1.1,
signing required) with enforcement on:

• Kerberos (AES256 / etype 18) and NTLM: session established,
  shares listed, C$ mounted, directory enumerated — no signature
  mismatch, no permission denied.
• go build ./... and go test ./pkg/third_party/smb2/ pass.