LU-14195 build: Adjust Makefile for Linux build changes.

LU-14195 build: Adjust Makefile for Linux build changes.

Since v5.10-rc1~51^2~19, "KBUILD_BUILTIN" has been unset
for module builds.  This means that "targets-for-builtin"
isn't built, and that is how "extra-y" is built.

So we need another way to force LUSTRE_KERNEL_TEST to be built.

Since v5.6-rc1~1^2~5 any target listed in "always-y" will always get
built.  So we can assign LUSTRE_KERNEL_TEST to this macro.

Assigning both macros is safe, even for those kernels which include
both in the list of targets.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I508b3710579c068dec93baf81ee383f3f03bd370
Reviewed-on: https://review.whamcloud.com/40907
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>