QLockFile: sync to disk to avoid leaving an empty lock file on crash

The two lines of code are the same as in QFSFileEnginePrivate::nativeSyncToDisk,
but we don't want to create a fileengine instance just for this.

qlockfile_win.cpp already calls FlushFileBuffers, only the Unix implementation
was missing the equivalent call.

Task-number: QTBUG-44771
Change-Id: I2253972857e4de9d27ef442c92983a1088d6f03e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qlockfile_unix.cpp

@@ -178,6 +178,13 @@ QLockFile::LockError QLockFilePrivate::tryLock_sys()
     // We hold the lock, continue.
     fileHandle = fd;
 
+    // Sync to disk if possible. Ignore errors (e.g. not supported).
+#if defined(_POSIX_SYNCHRONIZED_IO) && _POSIX_SYNCHRONIZED_IO > 0
+    fdatasync(fileHandle);
+#else
+    fsync(fileHandle);
+#endif
+
     return QLockFile::NoError;
 }