linux-user/syscall.c: Fix typo issue for using target_vec[i].iov_len instead of target_vec[i].iov_base

It is only a typo issue, need use tswapal(target_vec[i].iov_len) for the
len.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/syscall.c

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 7b6f482..310080c 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -1906,7 +1906,7 @@
     if (target_vec) {
         for (i = 0; i < count; i++) {
             abi_ulong base = tswapal(target_vec[i].iov_base);
-            abi_long len = tswapal(target_vec[i].iov_base);
+            abi_long len = tswapal(target_vec[i].iov_len);
             if (len < 0) {
                 break;
             }