If that's not the last reference, d_delete() will do d_drop().
If it is, dput() immediately after it will unhash the sucker
anyway, since ->d_delete() the method is always_delete_dentry().

IOW, there's no point trying to turn it into a negative hashed
dentry - it won't stick around anyway.  Just d_drop() it and be
done with that.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>