linuxkpi: Add shrinker_alloc() and shrinker_free()

They are used by the DRM drivers in Linux 6.7.

Bump FreeBSD_version because external drivers that use `struct
shrinker` will have to be recompiled.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48747