linux-user: fix socklen_t comparisons

On many systems, socklen_t is defined as unsigned. This means that
checks for negative values are not meaningful.

Fix by explicitly casting to a signed integer.

This also avoids some warnings with GCC flag -Wtype-limits.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 0ebe7e1..d44f512 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -1551,8 +1551,9 @@
     void *addr;
     abi_long ret;
 
-    if (addrlen < 0)
+    if ((int)addrlen < 0) {
         return -TARGET_EINVAL;
+    }
 
     addr = alloca(addrlen+1);
 
@@ -1570,8 +1571,9 @@
     void *addr;
     abi_long ret;
 
-    if (addrlen < 0)
+    if ((int)addrlen < 0) {
         return -TARGET_EINVAL;
+    }
 
     addr = alloca(addrlen);
 
@@ -1656,8 +1658,9 @@
     if (get_user_u32(addrlen, target_addrlen_addr))
         return -TARGET_EINVAL;
 
-    if (addrlen < 0)
+    if ((int)addrlen < 0) {
         return -TARGET_EINVAL;
+    }
 
     if (!access_ok(VERIFY_WRITE, target_addr, addrlen))
         return -TARGET_EINVAL;
@@ -1684,8 +1687,9 @@
     if (get_user_u32(addrlen, target_addrlen_addr))
         return -TARGET_EFAULT;
 
-    if (addrlen < 0)
+    if ((int)addrlen < 0) {
         return -TARGET_EINVAL;
+    }
 
     if (!access_ok(VERIFY_WRITE, target_addr, addrlen))
         return -TARGET_EFAULT;
@@ -1712,8 +1716,9 @@
     if (get_user_u32(addrlen, target_addrlen_addr))
         return -TARGET_EFAULT;
 
-    if (addrlen < 0)
+    if ((int)addrlen < 0) {
         return -TARGET_EINVAL;
+    }
 
     if (!access_ok(VERIFY_WRITE, target_addr, addrlen))
         return -TARGET_EFAULT;
@@ -1753,8 +1758,9 @@
     void *host_msg;
     abi_long ret;
 
-    if (addrlen < 0)
+    if ((int)addrlen < 0) {
         return -TARGET_EINVAL;
+    }
 
     host_msg = lock_user(VERIFY_READ, msg, len, 1);
     if (!host_msg)
@@ -1792,7 +1798,7 @@
             ret = -TARGET_EFAULT;
             goto fail;
         }
-        if (addrlen < 0) {
+        if ((int)addrlen < 0) {
             ret = -TARGET_EINVAL;
             goto fail;
         }