linuxkpi: Define __GFP_THISNODE

It is used to force the NUMA node to allocate from. This flag is
unimplemented for now because we don't have an implementation of
alloc_pages_node() yet.

The DRM TTM code started to use this flag in Linux 6.12.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 8f95998393f42f368fe22abd9a25c7fa4304e02a)