levels: don't sync directory in memory mode

[GanZiheng]

This will cause problem when performing compaction in memory mode, since we will never return tables built by compaction successfully due to the sync_dir returning error with empty path input.

Signed-off-by: GanZiheng [email protected]

[codecov[bot]]

Codecov Report

Merging #171 (1b8c19d) into master (a4e06f1) will increase coverage by 0.09%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
+ Coverage   89.54%   89.63%   +0.09%     
==========================================
  Files          39       39              
  Lines        8739     8741       +2     
==========================================
+ Hits         7825     7835      +10     
+ Misses        914      906       -8     

[GanZiheng]

What's the error

If we put entries enough to cause compaction in memory mode, we would get this error.

I tried to add test for this, but it will result in OOM in memory sanitizer test.