redhat: make selftests build install headers under tools/testing/selftests/usr/include

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2166738
Upstream Status: RHEL only

As can be seen from the build output of selftests, it tries to use the current headers
under tools/testing/selftests/usr/include

...
make[1]: Entering directory '/(...)/centos-stream-9/tools/testing/selftests/bpf'
clang  -g -Werror -D__TARGET_ARCH_x86 -mlittle-endian
 -I/(...)/centos-stream-9/tools/testing/selftests/bpf/tools/include
 -I/(...)/centos-stream-9/tools/testing/selftests/bpf
 -I/(...)/centos-stream-9/tools/include/uapi
 -I/(...)/centos-stream-9/tools/testing/selftests/usr/include
 -idirafter /usr/lib64/clang/15.0.1/include -idirafter /usr/local/include -idirafter /usr/include
 -Wno-compare-distinct-pointer-types -DENABLE_ATOMICS_TESTS -O2 -target bpf
 -c progs/cg_storage_multi_shared.c -mcpu=v3
 -o /(...)/centos-stream-9/tools/testing/selftests/bpf/cg_storage_multi_shared.o
...

However, when building without O=/KBUILD_OUTPUT= set, make doesn't install the uapi
headers from the source tree under tools/testing/selftests/, so the include above
using -I/(...)/centos-stream-9/tools/testing/selftests/usr/include does not work.
It falls back to the installed kernel-headers on the system. And if the installed
headers are outdated, build can fail, even if we patch the header like in the previous
commit. Specifying DEFAULT_INSTALL_HDR_PATH=0 tells the Makefile to end up installing
the uapi headers also at (...)/tools/testing/selftests directory, which we use here
as a workaround so kselftests can use it to get the latest header version.

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>

Author: Herton R. Krzesinski

redhat/kernel.spec.template

@@ -2323,7 +2323,7 @@ export BPFTOOL=$(pwd)/tools/bpf/bpftool/bpftool
 pushd tools/testing/selftests
 # We need to install here because we need to call make with ARCH set which
 # doesn't seem possible to do in the install section.
-%{make} %{?_smp_mflags} ARCH=$Arch V=1 TARGETS="bpf vm livepatch net net/forwarding net/mptcp netfilter tc-testing memfd" SKIP_TARGETS="" FORCE_TARGETS=1 INSTALL_PATH=%{buildroot}%{_libexecdir}/kselftests VMLINUX_H="${RPM_VMLINUX_H}" install
+%{make} %{?_smp_mflags} ARCH=$Arch V=1 TARGETS="bpf vm livepatch net net/forwarding net/mptcp netfilter tc-testing memfd" SKIP_TARGETS="" FORCE_TARGETS=1 INSTALL_PATH=%{buildroot}%{_libexecdir}/kselftests VMLINUX_H="${RPM_VMLINUX_H}" DEFAULT_INSTALL_HDR_PATH=0 install
 
 # 'make install' for bpf is broken and upstream refuses to fix it.
 # Install the needed files manually.