MIPS: Use force_sig when handling address errors.

When init is started it is SIGNAL_UNKILLABLE.  If it were to get an
address error, we would try to send it SIGBUS, but it would be ignored
and the faulting instruction restarted.  This results in an endless
loop.

We need to use force_sig() instead so it will actually die and give us
some useful information.

Reported-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>