We weren't telling the MDS what kind of unlink we were doing (unlink vs.
rmdir), so, for example, if you called rmdir() on a file, the MDS would
remove it and then the client VFS would return -ENODIR.  Not so good.

We send a 'mode' flag along with the unlink request now, that must be one of
S_IFDIR or S_IFREG.

I also fixed some unaligned structures in the MDS protocol, so if you update
one node you must UPDATE THEM ALL.

Minutiae:
- in the intent policy function, if mds_reint returns EISDIR or ENOTDIR, still
  go ahead and send back the file attributes
- in mds_reint_unlink, use the mode sent over the wire instead of the actual
  inode mode to determine which vfs unlink function to call