linux-user: Qemu handles `getsockopt` with NULL `optval` incorrectly
Description of problem:
In short call to `getsockopt(_, SOL_TCP, TCP_KEEPIDLE, NULL, _)` behaves differently on RISC-V Qemu than on x64 Linux. 
On Linux syscall returns 0, but on Qemu it fails with `"Bad address"`.
Apparently Qemu `getsockopt` implementation is more conservative about NULL `optval` argument than kernel implementation. However man permits passing NULL [link](https://man7.org/linux/man-pages/man2/setsockopt.2.html):

>  For getsockopt(), optlen is a value-result argument, initially
       containing the size of the buffer pointed to by optval, and
       modified on return to indicate the actual size of the value
       returned.  **If no option value is to be supplied** or returned,
       **optval may be NULL.**"

For me it sounds like accepting NULL without error (and x64 confirms that interpretation).
Steps to reproduce:
1. Use below toy program `getsockopt.c` and compile it without optimizations like:
```
    gcc -Wall -W -std=gnu11 -pedantic  getsockopt.c -o getsockopt
```

```
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <netinet/tcp.h>

static void fail_on_error(int error, const char *msg) {
    if (error < 0) {
        perror(msg);
        exit(errno);
    }
}

int main(int argc, char **argv) {
     int socketfd = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
     fail_on_error(socketfd, "socket error");
     uint8_t *option_value = NULL;
     int32_t len = 0;
     int32_t *option_len = &len;
     socklen_t opt_len = (socklen_t)*option_len;
     int status = getsockopt(socketfd, SOL_TCP, TCP_KEEPIDLE, option_value, &opt_len);
     fail_on_error(status, "getsockopt error");
     return 0;
}
```


2. Run program on Qemu and compare output with output from x64 build. In my case it looks like:
```
root@57646f544f3a:/runtime/programs# ./getsockopt-x64
root@57646f544f3a:/runtime/programs# ./getsockopt-riscv
getsockopt error: Bad address
```
Additional information:
I don't think issue is platform specific assuming Qemu `getsockopt` implementation that is actually running is here:
[link](https://github.com/qemu/qemu/blob/master/linux-user/syscall.c#L2522)

Looking at sources, I'm not sure why Qemu can't simply forward everything to kernel space 
instead doing extra sanity checks together with `optval` dereference attempt that eventually fails in one of `put_user*_` function: [link](https://github.com/qemu/qemu/blob/master/linux-user/syscall.c#L2753) 

Anyway, I think that interpretation of man quote is rather straightforward and Qemu `getsockopt` implementation should follow it.