staging/lustre: Unbreak ll_prep_inode iget result check

commit 020ecc6f

 ("staging: lustre: llite: Remove IS_ERR tests")
managed to break one of the converted checks by losing a dereference
causing the condition to be wrong and hence leading
to a crash later on in case of actual error.
IS_ERR_OR_NULL(*inode) got converted into !inode.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
CC: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>