fs/backing_file: fix wrong argument in callback

Commit 48b50624 ("backing-file: clean up the API") unintentionally
changed the argument in the ->accessed() callback from the user file to
the backing file.

Fixes: 48b50624 ("backing-file: clean up the API")
Reported-by:  <syzbot+8d1206605b05ca9a0e6a@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/linux-unionfs/67447b3c.050a0220.1cc393.0085.GAE@google.com/


Tested-by:  <syzbot+8d1206605b05ca9a0e6a@syzkaller.appspotmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20241126145342.364869-1-amir73il@gmail.com


Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>