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 <[e-mail]>
Fixes: 5340627e3fe0 ("riscv: add support for SECCOMP and SECCOMP_FILTER")
Signed-off-by: Paul Walmsley <[e-mail]>

arch/riscv/kernel/entry.S

···
 	 */
 	li t1, -1
 	beq a7, t1, ret_from_syscall_rejected
+	blt a7, t1, 1f
 	/* Call syscall */
 	la s0, sys_call_table
 	slli t0, a7, RISCV_LGPTR