riscv: reject invalid syscalls below -1

Running "stress-ng --enosys 4 -t 20 -v" showed a large number of kernel oops
with "Unable to handle kernel paging request at virtual address" message. This
happens when enosys stressor starts testing random non-valid syscalls.

I forgot to redirect any syscall below -1 to sys_ni_syscall.

With the patch kernel oops messages are gone while running stress-ng enosys
stressor.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Fixes: 5340627e ("riscv: add support for SECCOMP and SECCOMP_FILTER")
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>