On amd64, syscall.O_LARGEFILE is 0x0, making it be ignored as a
key. However, the kernel always passes 0x8000 to the FUSE server for
the OPEN call.

The previous behavior causes crashes on 386, because the flagname is
added twice.

Change-Id: Ibabcdfef4d90e4fa4d02963d45a4d4cf2cba1ea2