kbuild: skip per-CPU BTF generation for pahole v1.18-v1.21

Commit "mm/page_alloc: convert per-cpu list protection to local_lock" will
introduce a zero-sized per-CPU variable, which causes pahole to generate
invalid BTF.  Only pahole versions 1.18 through 1.21 are impacted, as
before 1.18 pahole doesn't know anything about per-CPU variables, and 1.22
contains the proper fix for the issue.

Luckily, pahole 1.18 got --skip_encoding_btf_vars option disabling BTF
generation for per-CPU variables in anticipation of some unanticipated
problems.  So use this escape hatch to disable per-CPU var BTF info on
those problematic pahole versions.  Users relying on availability of
per-CPU var BTFs would need to upgrade to pahole 1.22+, but everyone won't
notice any regressions.

Link: https://lkml.kernel.org/r/20210530002536.3193829-1-andrii@kernel.org
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Michal Suchanek <msuchanek@suse.de>
Cc: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: anakryiko

scripts/link-vmlinux.sh

@@ -235,6 +235,10 @@ gen_btf()
 
 	vmlinux_link ${1}
 
+	if [ "${pahole_ver}" -ge "118" ] && [ "${pahole_ver}" -le "121" ]; then
+		# pahole 1.18 through 1.21 can't handle zero-sized per-CPU vars
+		extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_vars"
+	fi
 	if [ "${pahole_ver}" -ge "121" ]; then
 		extra_paholeopt="${extra_paholeopt} --btf_gen_floats"
 	fi