io/io: fix for ROOT-8516 (corruption in file writing)

When the write-cache required flushing to file and the length of the buffer
to be written exceed the size of the cache buffer, requiring
direct-write-to-file, the offset (seek position) was incorrectly left to
the one of the last basket in the flushed cache.

io/io/src/TFileCacheWrite.cxx

@@ -133,6 +133,7 @@ Int_t TFileCacheWrite::WriteBuffer(const char *buf, Long64_t pos, Int_t len)
       if (len >= fBufferSize) {
          //buffer larger than the cache itself: direct write to file
          fRecursive = kTRUE;
+         fFile->Seek(pos); // Flush may have changed this
          if (fFile->WriteBuffer(buf,len)) return -1;  // failure
          fRecursive = kFALSE;
          return 1;