hurd: Fix vm_size_t incoherencies

In gnumach, 3e1702a65fb3 ("add rpc_versions for vm types") changed the type
of vm_size_t, making it always a unsigned long. This made it incompatible on
x86 with size_t. Even if we may want to revert it to unsigned int, it's
better to fix the types of parameters according to the .defs files.

sysdeps/mach/hurd/sendmsg.c

@@ -47,7 +47,7 @@ __libc_sendmsg (int fd, const struct msghdr *message, int flags)
   } data = { .ptr = NULL };
   char data_buf[2048];
   mach_msg_type_number_t len;
-  mach_msg_type_number_t amount;
+  vm_size_t amount;
   int dealloc = 0;
   int socketrpc = 0;
   int i;