Fix a bug that DB stats of old file become zero during compaction

- Before moving docs from old file to new file, since the new file is
empty, fdb_set_file_header() appends a new header whose stats are
zero-filled into the old file.

- Since DB header will not be appended to the new file before compaction
is done, fdb_set_file_header() will be invoked only for the old file.
Hence, we don't need to consider 'file->new_file'.

Change-Id: I7d6a185a7b911f13ecbf29fff3274b0a71cd7b58