staging: lustre: honor error code from ll_iget().

Commit 020ecc6f ("staging: lustre: llite: Remove IS_ERR tests")
changed ll_prep_inode to assume any error from ll_iget() meant
-ENOMEM because at that time it only returned NULL for errors.
Commit c3397e7e

 ("staging: lustre: llite: add error handler in
inode prepare phase") changed ll_iget() to once again return
meaningful codes, but nobody told ll_prep_inode().

So change ll_prep_inode() back to using PTR_ERR(*inode).

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>