riscv: extable: fix err reg writing in dedicated uaccess handler

Mayuresh reported commit 20802d8d ("riscv: extable: add a dedicated
uaccess handler") breaks the writev02 test case in LTP. This is due to
the err reg isn't correctly set with the errno(-EFAULT in writev02
case). First of all, the err and zero regs are reg numbers rather than
reg offsets in struct pt_regs; Secondly, regs_set_gpr() should write
the regs when offset isn't zero(zero means epc)

Fix it by correcting regs_set_gpr() logic and passing the correct reg
offset to it.
Reported-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Fixes: 20802d8d ("riscv: extable: add a dedicated uaccess handler")
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/mm/extable.c

@@ -33,7 +33,7 @@ static inline void regs_set_gpr(struct pt_regs *regs, unsigned int offset,
 	if (unlikely(offset > MAX_REG_OFFSET))
 		return;
 
-	if (!offset)
+	if (offset)
 		*(unsigned long *)((unsigned long)regs + offset) = val;
 }
 
@@ -43,8 +43,8 @@ static bool ex_handler_uaccess_err_zero(const struct exception_table_entry *ex,
 	int reg_err = FIELD_GET(EX_DATA_REG_ERR, ex->data);
 	int reg_zero = FIELD_GET(EX_DATA_REG_ZERO, ex->data);
 
-	regs_set_gpr(regs, reg_err, -EFAULT);
-	regs_set_gpr(regs, reg_zero, 0);
+	regs_set_gpr(regs, reg_err * sizeof(unsigned long), -EFAULT);
+	regs_set_gpr(regs, reg_zero * sizeof(unsigned long), 0);
 
 	regs->epc = get_ex_fixup(ex);
 	return true;