orangefs: Fix kmemleak in orangefs_prepare_debugfs_help_string()

When insert and remove the orangefs module, then debug_help_string will
be leaked:

  unreferenced object 0xffff8881652ba000 (size 4096):
    comm "insmod", pid 1701, jiffies 4294893639 (age 13218.530s)
    hex dump (first 32 bytes):
      43 6c 69 65 6e 74 20 44 65 62 75 67 20 4b 65 79  Client Debug Key
      77 6f 72 64 73 20 61 72 65 20 75 6e 6b 6e 6f 77  words are unknow
    backtrace:
      [<0000000004e6f8e3>] kmalloc_trace+0x27/0xa0
      [<0000000006f75d85>] orangefs_prepare_debugfs_help_string+0x5e/0x480 [orangefs]
      [<0000000091270a2a>] _sub_I_65535_1+0x57/0xf70 [crc_itu_t]
      [<000000004b1ee1a3>] do_one_initcall+0x87/0x2a0
      [<000000001d0614ae>] do_init_module+0xdf/0x320
      [<00000000efef068c>] load_module+0x2f98/0x3330
      [<000000006533b44d>] __do_sys_finit_module+0x113/0x1b0
      [<00000000a0da6f99>] do_syscall_64+0x35/0x80
      [<000000007790b19b>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

When remove the module, should always free debug_help_string. Should
always free the allocated buffer when change the free_debug_help_string.
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>

fs/orangefs/orangefs-debugfs.c

@@ -222,6 +222,8 @@ static void orangefs_kernel_debug_init(void)
 void orangefs_debugfs_cleanup(void)
 {
 	debugfs_remove_recursive(debug_dir);
+	kfree(debug_help_string);
+	debug_help_string = NULL;
 }
 
 /* open ORANGEFS_KMOD_DEBUG_HELP_FILE */
@@ -671,6 +673,7 @@ int orangefs_prepare_debugfs_help_string(int at_boot)
 		memset(debug_help_string, 0, DEBUG_HELP_STRING_SIZE);
 		strlcat(debug_help_string, new, string_size);
 		mutex_unlock(&orangefs_help_file_lock);
+		kfree(new);
 	}
 
 	rc = 0;