Fix extracting pid form bpf_get_current_pid_tgid lizrice#35

According to `man bpf-helpers` pid is located in the lower 32 bits of the return value. Pid can be easily extracted by using an and operation with `0xFFFFFFFF` 

Signed-off-by: Ali Javidi Ghasr <[email protected]>

chapter2/hello-buffer.py

@@ -15,7 +15,7 @@
    struct data_t data = {}; 
    char message[12] = "Hello World";
  
-   data.pid = bpf_get_current_pid_tgid() >> 32;
+   data.pid = bpf_get_current_pid_tgid() & 0xFFFFFFFF;
    data.uid = bpf_get_current_uid_gid() & 0xFFFFFFFF;
    
    bpf_get_current_comm(&data.command, sizeof(data.command));