seq_file: fix passing wrong private data

DEFINE_PROC_SHOW_ATTRIBUTE() is supposed to be used to define a series
of functions and variables to register proc file easily. And the users
can use proc_create_data() to pass their own private data and get it
via seq->private in the callback. Unfortunately, the proc file system
use PDE_DATA() to get private data instead of inode->i_private. So fix
it. Fortunately, there only one user of it which does not pass any
private data, so this bug does not break any in-tree codes.

Link: https://lkml.kernel.org/r/20211029032638.84884-1-songmuchun@bytedance.com
Fixes: 97a32539

 ("proc: convert everything to "struct proc_ops"")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Florent Revest <revest@chromium.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>