lustre: switch ll_intent_file_open() to struct dentry *

... because fake struct file is wrong.

ll_prep_inode() in there is an atrocity - despite passing a pointer to
inode by address, it actually only uses the value when that value is
non-NULL, as it will be here.

Oh, and ->d_parent of anything is never NULL.

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