selftests/net: fix typo in tcp_mmap

selftests/net: fix typo in tcp_mmap

kernel test robot reported the following warning:

All warnings (new ones prefixed by >>):

   tcp_mmap.c: In function 'child_thread':
>> tcp_mmap.c:211:61: warning: 'lu' may be used uninitialized in this function [-Wmaybe-uninitialized]
     211 |                         zc.length = min(chunk_size, FILE_SZ - lu);

We want to read FILE_SZ bytes, so the correct expression
should be (FILE_SZ - total)

Fixes: 5c5945dc695c ("selftests/net: Add SHA256 computation over data sent in tcp_mmap")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202304042104.UFIuevBp-lkp@intel.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Xiaoyan Li <lixiaoyan@google.com>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230405071556.1019623-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>