linuxkpi: Add Linux 6.7 get_file_rcu() variant

[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.

Reviewed by: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48756