riscv: kprobes: allow writing to x0

Instructions can write to x0, so we should simulate these instructions
normally.

Currently, the kernel hangs if an instruction who writes to x0 is
simulated.

Fixes: c22b0bcb ("riscv: Add kprobes supported")
Cc: stable@vger.kernel.org
Signed-off-by: Nam Cao <namcaov@gmail.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Acked-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230829182500.61875-1-namcaov@gmail.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/probes/simulate-insn.c

@@ -24,7 +24,7 @@ static inline bool rv_insn_reg_set_val(struct pt_regs *regs, u32 index,
 				       unsigned long val)
 {
 	if (index == 0)
-		return false;
+		return true;
 	else if (index <= 31)
 		*((unsigned long *)regs + index) = val;
 	else