linux-user: Return target error number in do_fork()

Whilst calls to do_fork() are wrapped in get_errno() this does not
translate return values.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 75ac32c..c25ffd8 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -4626,8 +4626,9 @@
         pthread_mutex_unlock(&clone_lock);
     } else {
         /* if no CLONE_VM, we consider it is a fork */
-        if ((flags & ~(CSIGNAL | CLONE_NPTL_FLAGS2)) != 0)
-            return -EINVAL;
+        if ((flags & ~(CSIGNAL | CLONE_NPTL_FLAGS2)) != 0) {
+            return -TARGET_EINVAL;
+        }
         fork_start();
         ret = fork();
         if (ret == 0) {