staging: lustre: llite: Remove IS_ERR tests

ll_iget() has been modified to always return NULL in case of error and
not a ERR_PTR.

Consequently, remove unnecessary IS_ERR tests as now ll_iget()
function can never return a ERR_PTR.

Additionally, reinitialising root and inode to NULL has been removed
since they have already been tested for the same.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>