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/dep2.c

@@ -5,17 +5,17 @@
 #include <linux/module.h>
 
 extern u32 lkmc_dep;
-static struct dentry *debugfs_file;
+static char *debugfs_filename = "lkmc_dep2";
 
 static int myinit(void)
 {
-	debugfs_file = debugfs_create_u32("lkmc_dep2", S_IRUSR | S_IWUSR, NULL, &lkmc_dep);
+	debugfs_create_u32(debugfs_filename, S_IRUSR | S_IWUSR, NULL, &lkmc_dep);
 	return 0;
 }
 
 static void myexit(void)
 {
-	debugfs_remove(debugfs_file);
+	debugfs_remove(debugfs_lookup(debugfs_filename, NULL));
 }
 
 module_init(myinit)