multisig: fix #8537 seed restore (suggestions by @UkoeHB)

- spend secret key is no longer the sum of multisig key shares;
no need to check that is the case upon restore.
- restoring a multisig wallet from multisig info means that the
wallet must have already completed all setup rounds. Upon restore,
set the number of rounds completed accordingly.

src/multisig/multisig_account.cpp

@@ -127,7 +127,7 @@ namespace multisig
   bool multisig_account::multisig_is_ready() const
   {
     if (main_kex_rounds_done())
-      return m_kex_rounds_complete >= multisig_kex_rounds_required(m_signers.size(), m_threshold) + 1;
+      return m_kex_rounds_complete >= multisig_setup_rounds_required(m_signers.size(), m_threshold);
     else
       return false;
   }
@@ -200,4 +200,11 @@ namespace multisig
     return num_signers - threshold + 1;
   }
   //----------------------------------------------------------------------------------------------------------------------
+  // EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  std::uint32_t multisig_setup_rounds_required(const std::uint32_t num_signers, const std::uint32_t threshold)
+  {
+    return multisig_kex_rounds_required(num_signers, threshold) + 1;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
 } //namespace multisig

src/multisig/multisig_account.h

@@ -245,4 +245,13 @@ namespace multisig
   * return: number of kex rounds required
   */
   std::uint32_t multisig_kex_rounds_required(const std::uint32_t num_signers, const std::uint32_t threshold);
+
+  /**
+  * brief: multisig_setup_rounds_required - The number of setup rounds required to produce an M-of-N shared key.
+  *    - A participant must complete all kex rounds and 1 initialization round.
+  * param: num_signers - number of participants in multisig (N)
+  * param: threshold - threshold of multisig (M)
+  * return: number of setup rounds required
+  */
+  std::uint32_t multisig_setup_rounds_required(const std::uint32_t num_signers, const std::uint32_t threshold);
 } //namespace multisig

src/multisig/multisig_account_kex_impl.cpp

@@ -74,7 +74,7 @@ namespace multisig
       "Multisig threshold may not be larger than number of signers.");
     CHECK_AND_ASSERT_THROW_MES(threshold > 0, "Multisig threshold must be > 0.");
     CHECK_AND_ASSERT_THROW_MES(round > 0, "Multisig kex round must be > 0.");
-    CHECK_AND_ASSERT_THROW_MES(round <= multisig_kex_rounds_required(num_signers, threshold) + 1,
+    CHECK_AND_ASSERT_THROW_MES(round <= multisig_setup_rounds_required(num_signers, threshold),
       "Trying to process multisig kex for an invalid round.");
   }
   //----------------------------------------------------------------------------------------------------------------------