efivarfs: Defer PM notifier registration until .fill_super

syzbot reports an issue that turns out to be caused by the fact that the
efivarfs PM notifier may be invoked before the efivarfs_fs_info::sb
field is populated, resulting in a NULL deference.

So defer the registration until efivarfs_fill_super() is invoked.

Reported-by: syzbot+00d13e505ef530a45100@syzkaller.appspotmail.com
Tested-by: syzbot+00d13e505ef530a45100@syzkaller.appspotmail.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: ardbiesheuvel

fs/efivarfs/super.c

@@ -367,6 +367,8 @@ static int efivarfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	if (err)
 		return err;
 
+	register_pm_notifier(&sfi->pm_nb);
+
 	return efivar_init(efivarfs_callback, sb, true);
 }
 
@@ -552,7 +554,6 @@ static int efivarfs_init_fs_context(struct fs_context *fc)
 
 	sfi->pm_nb.notifier_call = efivarfs_pm_notify;
 	sfi->pm_nb.priority = 0;
-	register_pm_notifier(&sfi->pm_nb);
 
 	return 0;
 }