linux-user: write(fd, NULL, 0) parity with linux's treatment of same

Bring linux-user write(2) handling into line with linux for the case
of a 0-byte write with a NULL buffer. Based on a patch originally
written by Zhuowei Zhang.

Addresses https://bugs.launchpad.net/qemu/+bug/1716292.

>From Zhuowei Zhang's patch (https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg08073.html):

    Linux returns success for the special case of calling write with a
    zero-length NULL buffer: compiling and running

    int main() {
       ssize_t ret = write(STDOUT_FILENO, NULL, 0);
       fprintf(stderr, "write returned %ld\n", ret);
       return 0;
    }

    gives "write returned 0" when run directly, but "write returned
    -1" in QEMU.

    This commit checks for this situation and returns success if
    found.

Subsequent discussion raised the following questions (and my answers):

 - Q. Should TARGET_NR_read pass through to safe_read in this
      situation too?
   A. I'm wary of changing unrelated code to the specific problem I'm
      addressing. TARGET_NR_read is already consistent with Linux for
      this case.

 - Q. Do pread64/pwrite64 need to be changed similarly?
   A. Experiment suggests not: both linux and linux-user yield -1 for
      NULL 0-length reads/writes.

Signed-off-by: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20180908182205.GB409@mornington.dcs.gla.ac.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
(cherry picked from commit 58cfa6c2e6)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

linux-user/syscall.c

@@ -8077,6 +8077,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
         }
         break;
     case TARGET_NR_write:
+        if (arg2 == 0 && arg3 == 0) {
+            return get_errno(safe_write(arg1, 0, 0));
+        }
         if (!(p = lock_user(VERIFY_READ, arg2, arg3, 1)))
             goto efault;
         if (fd_trans_target_to_host_data(arg1)) {