Fix dereference NULL pointer with proc_lseek

Since the operations are static storage duration, the pointer in
operation structures will initialize with NULL. But, the kernel
doesn't check whether the pointer is NULL or not when calling it.

Related Discussion:
- https://github.com/sysprog21/lkmpg/issues/165
- https://github.com/sysprog21/lkmpg/issues/160

Close #165

examples/sleep.c

@@ -169,6 +169,7 @@ static const struct proc_ops file_ops_4_our_proc_file = {
     .proc_write = module_input, /* "write" to the file */
     .proc_open = module_open, /* called when the /proc file is opened */
     .proc_release = module_close, /* called when it's closed */
+    .proc_lseek = noop_llseek, /* return file->f_pos */
 };
 #else
 static const struct file_operations file_ops_4_our_proc_file = {
@@ -176,6 +177,7 @@ static const struct file_operations file_ops_4_our_proc_file = {
     .write = module_input,
     .open = module_open,
     .release = module_close,
+    .llseek = noop_llseek,
 };
 #endif