Merge #20140: Restore compatibility with old CSubNet serialization

886be97af5 Ignore incorrectly-serialized banlist.dat entries (Pieter Wuille)
883cea7dea Restore compatibility with old CSubNet serialization (Pieter Wuille)

Pull request description:

  #19628 changed CSubNet for IPv4 netmasks, using the first 4 bytes of `netmask` rather than the last 4 to store the actual mask. Unfortunately, CSubNet objects are serialized on disk in banlist.dat, breaking compatibility with existing banlists (and bringing them into an inconsistent state where entries reported in `listbanned` cannot be removed).

  Fix this by reverting to the old format (just for serialization). Also add a sanity check to the deserializer so that nonsensical banlist.dat entries are ignored (which would otherwise be possible if someone added IPv4 entries after #19628 but without this PR).

  Reported by Greg Maxwell.

ACKs for top commit:
  laanwj:
    Code review ACK 886be97af5
  vasild:
    ACK 886be97af

Tree-SHA512: d3fb91e8ecd933406e527187974f22770374ee2e12a233e7870363f52ecda471fb0b7bae72420e8ff6b6b1594e3037a5115984c023dbadf38f86aeaffcd681e7

src/netaddress.cpp

@@ -1109,6 +1109,17 @@ bool CSubNet::IsValid() const
     return valid;
 }
 
+bool CSubNet::SanityCheck() const
+{
+    if (!(network.IsIPv4() || network.IsIPv6())) return false;
+
+    for (size_t x = 0; x < network.m_addr.size(); ++x) {
+        if (network.m_addr[x] & ~netmask[x]) return false;
+    }
+
+    return true;
+}
+
 bool operator==(const CSubNet& a, const CSubNet& b)
 {
     return a.valid == b.valid && a.network == b.network && !memcmp(a.netmask, b.netmask, 16);