ncpfs: get rid of d_validate() nonsense

What we want is to have non-counting references to children in
pagecache of parent directory, and avoid picking them after a child
has been freed.  Fine, so let's just have ->d_prune() clear
parent's inode "has directory contents in page cache" flag.
That way we don't need ->d_fsdata for storing offsets, so we can
use it as a quick and dirty "is it referenced from page cache"
flag.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>