linux-user/mmap.c: Always zero MAP_ANONYMOUS memory in mmap_frag()

When mapping MAP_ANONYMOUS memory fragments, still need notice about to
set it zero, or it will cause issues.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/mmap.c

@@ -186,10 +186,12 @@ static int mmap_frag(abi_ulong real_start,
         if (prot_new != (prot1 | PROT_WRITE))
             mprotect(host_start, qemu_host_page_size, prot_new);
     } else {
-        /* just update the protection */
         if (prot_new != prot1) {
             mprotect(host_start, qemu_host_page_size, prot_new);
         }
+        if (prot_new & PROT_WRITE) {
+            memset(g2h(start), 0, end - start);
+        }
     }
     return 0;
 }