fs: make file_dentry() a simple accessor

file_dentry() is a relic from the days that overlayfs was using files with
a "fake" path, meaning, f_path on overlayfs and f_inode on underlying fs.

In those days, file_dentry() was needed to get the underlying fs dentry
that matches f_inode.

Files with "fake" path should not exist nowadays, so make file_dentry() a
simple accessor and use an assertion to make sure that file_dentry() was
not papering over filesystem bugs.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20240202110132.1584111-2-amir73il@gmail.com


Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>