m68k: invoke oom-killer from page fault

As explained in commit 1c0fe6e3, we want to call the architecture independent
oom killer when getting an unexplained OOM from handle_mm_fault, rather than
simply killing current.

Cc: linux-m68k@lists.linux-m68k.org
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-arch@vger.kernel.org
Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: David Rientjes <rientjes@google.com>
[Geert] Kill 2 introduced compiler warnings
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>