thread-safety: fix annotations with REVERSE_LOCK

Without proper annotations, clang thinks that mutexes are still held for the
duration of a reverse_lock. This could lead to subtle bugs as
EXCLUSIVE_LOCKS_REQUIRED(foo) passes when it shouldn't.

As mentioned in the docs [0], clang's thread-safety analyzer is unable to deal
with aliases of mutexes, so it is not possible to use the lock's copy of the
mutex for that purpose. Instead, the original mutex needs to be passed back to
the reverse_lock for the sake of thread-safety analysis, but it is not actually
used otherwise.

[0]: https://clang.llvm.org/docs/ThreadSafetyAnalysis.html

src/node/interfaces.cpp

@@ -443,7 +443,7 @@ bool FillBlock(const CBlockIndex* index, const FoundBlock& block, UniqueLock<Rec
     if (block.m_locator) { *block.m_locator = GetLocator(index); }
     if (block.m_next_block) FillBlock(active[index->nHeight] == index ? active[index->nHeight + 1] : nullptr, *block.m_next_block, lock, active, blockman);
     if (block.m_data) {
-        REVERSE_LOCK(lock);
+        REVERSE_LOCK(lock, cs_main);
         if (!blockman.ReadBlock(*block.m_data, *index)) block.m_data->SetNull();
     }
     block.found = true;