Reduce severity of too many levels log message (#5742)

Summary:
This condition is now a normal occurrence during write burst so there is
no need to warn the user about it. Here is a scenario where it happens
under completely normal conditions.

* Initially we have a DB of three levels (L0, L1, and L2) that is stable, i.e., compaction scores are all less than one.
* Now a write burst comes along. At first L0 blows up a bit in size as compaction hasn't had a chance to catch up.
* As a result of the above, `base_bytes_min` also increases since it is based on L0 size as of https://github.com/facebook/rocksdb/issues/4338
* If `base_bytes_min` increased enough (i.e., to be larger than L1), then we are shown the warning that the DB has more levels than necessary.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5742

Differential Revision: D17059221

fbshipit-source-id: e4a31d6eea42089a8d273095f19653991bd91bea

db/version_set.cc

@@ -3215,7 +3215,7 @@ void VersionStorageInfo::CalculateBaseBytes(const ImmutableCFOptions& ioptions,
         // base_bytes_min. We set it be base_bytes_min.
         base_level_size = base_bytes_min + 1U;
         base_level_ = first_non_empty_level;
-        ROCKS_LOG_WARN(ioptions.info_log,
+        ROCKS_LOG_INFO(ioptions.info_log,
                        "More existing levels in DB than needed. "
                        "max_bytes_for_level_multiplier may not be guaranteed.");
       } else {