bdi: Fix use after free bug in debugfs_remove()

bdi: Fix use after free bug in debugfs_remove()

syzbot is reporting use after free bug in debugfs_remove() [1].

This is because fault injection made memory allocation for
debugfs_create_file() from bdi_debug_register() from bdi_register_va()
fail and continued with setting WB_registered. But when debugfs_remove()
is called from debugfs_remove(bdi->debug_dir) from bdi_debug_unregister()
 from bdi_unregister() from release_bdi() because WB_registered was set
by bdi_register_va(), IS_ERR_OR_NULL(bdi->debug_dir) == false despite
debugfs_remove(bdi->debug_dir) was already called from bdi_register_va().

Fix this by making IS_ERR_OR_NULL(bdi->debug_dir) == true.

[1] https://syzkaller.appspot.com/bug?id=5ab4efd91a96dcea9b68104f159adf4af2a6dfc1

Signed-off-by: Tetsuo Handa <[email protected]>
Reported-by: syzbot <[email protected]>
Fixes: 97f07697932e6faf ("bdi: convert bdi_debug_register to int")
Cc: weiping zhang <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>