fat: fix uninitialized field in nostale filehandles

In the Linux kernel, the following vulnerability has been resolved: fat: fix uninitialized field in nostale filehandles When fat_encode_fh_nostale() encodes file handle without a parent it stores only first 10 bytes of the file handle. However the length of the file handle must be a multiple of 4 so the file handle is actually 12 bytes long and the last two bytes remain uninitialized. This is not great at we potentially leak uninitialized information with the handle to userspace. Properly initialize the full handle length.