prevent double lock for 'm_banned_mutex'

`BanMan::SweepBanned()` is only called internally in `src/banman.cpp`.
`BanMan::GetBanned()` locks 'm_banned_mutex' and then calls `BanMan::SweepBanned()`
without releasing the lock.
This commit changes `BanMan::GetBanned()` to lock 'm_banned_mutex' only after
`BanMan::SweepBanned()` releases it.

src/banman.cpp

@@ -159,9 +159,9 @@ bool BanMan::Unban(const CSubNet& sub_net)
 
 void BanMan::GetBanned(banmap_t& banmap)
 {
-    LOCK(m_banned_mutex);
     // Sweep the banlist so expired bans are not returned
     SweepBanned();
+    LOCK(m_banned_mutex);
     banmap = m_banned; //create a thread safe copy
 }