Merge patch series "backing_file accessors cleanup"

Amir Goldstein <amir73il@gmail.com> says:

As promissed, here is the backing_file accessors cleanup that
was dicussed on the overlayfs pr [1].

I have kept the ovl patch separate from the vfs patch, so that
the vfs patch could be backported to stable kernels, because
the ovl patch depends on master of today.

* patches from https://lore.kernel.org/20250607115304.2521155-1-amir73il@gmail.com:
  ovl: remove unneeded non-const conversion
  fs: constify file ptr in backing_file accessor helpers

Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxgFJCikAi4o4e9vzXTH=cUQGyvoo+cpdtfmBwJzutSCzw@mail.gmail.com/
Link: https://lore.kernel.org/20250607115304.2521155-1-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>