arc: don't leak bits of kernel stack into coredump

On faulting sigreturn we do get SIGSEGV, all right, but anything
we'd put into pt_regs could end up in the coredump.  And since
__copy_from_user() never zeroed on arc, we'd better bugger off
on its failure without copying random uninitialized bits of
kernel stack into pt_regs...

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>