linux-user/syscall.c: lock_iovec: unlock vec[i] in failure processing code block

When failure occurs during locking of vec[i], we also need to unlock all
already locked vec[i] in failure processing code block before return.

Code in unlock_user() checks vec[i].iov_base for NULL, so there's no
need not check it .

If error is EFAULT when "i == 0", vec[i].iov_base is NULL, we can just
skip it, so can still use "while (--i >= 0)" loop condition.
Signed-off-by: NChen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

linux-user/syscall.c

@@ -1883,6 +1883,11 @@ static struct iovec *lock_iovec(int type, abi_ulong target_addr,
     return vec;
 
  fail:
+    while (--i >= 0) {
+        if (tswapal(target_vec[i].iov_len) > 0) {
+            unlock_user(vec[i].iov_base, tswapal(target_vec[i].iov_base), 0);
+        }
+    }
     unlock_user(target_vec, target_addr, 0);
  fail2:
     free(vec);