fix kernel modules build after updating linux to v5.9.2

- `dep.c` and `dep2.c`: `debugfs_create_u32` does not return anymore, not
  sure why:
  https://unix.stackexchange.com/questions/593983/creating-a-debugfs-file-that-is-used-to-read-write-u32-value/621282#621282
  So just doing `debugfs_lookup` for now
- vermagic.c:
  51161bfc66
  prevents its usage in v5.8. Just migrating to `init_utsname` for now
- procfs.c: `struct file_operations` moved to a new `struct proc_ops` at:
  b567e07513
- myprintk.c: `pr_warning` dropped for `pr_warn`:
  61ff72f401
- `poll.c`: `kthread_func` is not defined in kernel, prefix with lkmc to
  avoid conflict

Fix https://github.com/cirosantilli/linux-kernel-module-cheat/issues/136

Fix https://github.com/cirosantilli/linux-kernel-module-cheat/issues/137

kernel_modules/debugfs.c

@@ -33,17 +33,12 @@ static const struct file_operations fops = {
 
 static int myinit(void)
 {
-	struct dentry *file;
 	dir = debugfs_create_dir("lkmc_debugfs", 0);
 	if (!dir) {
 		pr_alert("debugfs_create_dir failed");
 		return -1;
 	}
-	file = debugfs_create_u32("myfile", S_IRUSR | S_IWUSR, dir, &value);
-	if (!file) {
-		pr_alert("debugfs_create_u32 failed");
-		return -1;
-	}
+	debugfs_create_u32("myfile", S_IRUSR | S_IWUSR, dir, &value);
 
 	/* Created on the toplevel of the debugfs mount,
 	 * and with explicit fops instead of a fixed integer value.