Updated the reference for the assertion about munmap.

The link points here:

    http://sourceforge.net/p/fuse/mailman/message/33627382/

This is a fuse-devel thread titled "munmap(2) and fuse flush requests"
by me on 2015-03-23.

file_system.go

@@ -243,9 +243,8 @@ type FileSystem interface {
 	// is not.
 	//
 	// Note that one potentially significant case where this is *not* called is
-	// munmap(2). (Cf. http://goo.gl/7n1r9X, fuse-devel mailing list thread from
-	// Han-Wen Nienhuys on 2014-10-08.) Even if users close(2) after writing to
-	// an mmap'd file, on OS X the contents are not immediately flushed (cf.
+	// munmap(2) (cf. http://goo.gl/j8B9g0). Even if users close(2) after writing
+	// to an mmap'd file, on OS X the contents are not immediately flushed (cf.
 	// https://github.com/osxfuse/osxfuse/issues/202).
 	//
 	// Because of cases like dup2(2), calls to FlushFile are not necessarily one