Merge bitcoin/bitcoin#26078: p2p: return CSubNet in LookupSubNet

fb3e812277 p2p: return `CSubNet` in `LookupSubNet` (brunoerg)

Pull request description:

  Analyzing the usage of `LookupSubNet`, noticed that most cases uses check if the subnet is valid by calling `subnet.IsValid()`, and the boolean returned by `LookupSubNet` hasn't been used so much, see:
  29d540b7ad/src/httpserver.cpp (L172-L174)
  29d540b7ad/src/net_permissions.cpp (L114-L116)

  It makes sense to return `CSubNet` instead of `bool`.

ACKs for top commit:
  achow101:
    ACK fb3e812277
  vasild:
    ACK fb3e812277
  theStack:
    Code-review ACK fb3e812277
  stickies-v:
    Concept ACK, but Approach ~0 (for now). Reviewed the code (fb3e812277) and it all looks good to me.

Tree-SHA512: ba50d6bd5d58dfdbe1ce1faebd80dd8cf8c92ac53ef33519860b83399afffab482d5658cb6921b849d7a3df6d5cea911412850e08f3f4e27f7af510fbde4b254

src/test/fuzz/netbase_dns_lookup.cpp

@@ -59,9 +59,6 @@ FUZZ_TARGET(netbase_dns_lookup)
         assert(!resolved_service.IsInternal());
     }
     {
-        CSubNet resolved_subnet;
-        if (LookupSubNet(name, resolved_subnet)) {
-            assert(resolved_subnet.IsValid());
-        }
+        (void)LookupSubNet(name);
     }
 }