ARC: mm: fix uninitialised signal code in do_page_fault

Commit 15773ae9 ("signal/arc: Use force_sig_fault where
appropriate") introduced undefined behaviour by leaving si_code
unitiailized and leaking random kernel values to user space.

Fixes: 15773ae9 ("signal/arc: Use force_sig_fault where appropriate")
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>