nios2: mm: do not invoke OOM killer on kernel fault OOM

Follow commit 87134102.

Kernel faults are expected to handle OOM conditions gracefully (gup,
uaccess etc.), so they should never invoke the OOM killer.  Reserve
this for faults triggered in user context when it is the only option.

Signed-off-by: Ley Foon Tan <lftan@altera.com>