linuxkpi: Add show_fdinfo() to struct file_operations

We never call this field but it allows to compile drivers without
modifications.

This is used by the DRM drivers for some time, but another change part
of the update to Linux 6.8 led to compile them with CONFIG_PROC_FS.
This connected code that uses this struct field.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49069