linuxkpi: Add Linux 6.7 `get_file_rcu()` variant

[dumbbell]

Summary

Why

In Linux 6.7, the signature of get_file_rcu() changed significantly, going from:

bool get_file_rcu(struct linux_file *f);

... to:

struct linux_file * get_file_rcu(struct linux_file **f);

I.e., both the argument and the return value changed in an incompatible way.

This is used by the i915 DRM driver.

How

This patch introduces the variant and hide the new prototype behind LINUXKPI_VERSION >= 60700.

This is part of the update of DRM drivers to Linux 6.7.

Sponsored by: The FreeBSD Foundation