- hpfs_unlink() should not try to truncate the victim unless it's a
   regular file; truncate will not help for anything else and it will
   screw the page cache if victim happens to be a block device.
 - network filesystems should *not* invalidate page cache of block
   device node when inode gets invalidated; fixed, added a new helper -
   invalidate_remote_inode(). 
 - nfs setattr syncs the file before sending SETATTR to server; that
   makes a lot of sense for regular files, but not for anything else.
   Fixed.