mainline inclusion
from mainline-5.13-rc4
commit 9683e577
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9683e5775c75097c46bd24e65411b16ac6c6cbb3

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Avoids a segv if btf isn't present. Seen on the call path
__bpf_object__open calling bpf_object__collect_externs.

Fixes: 5bd022ec (libbpf: Support extern kernel function)
Suggested-by: NStanislav Fomichev <sdf@google.com>
Suggested-by: NPetar Penkov <ppenkov@google.com>
Signed-off-by: NIan Rogers <irogers@google.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210504234910.976501-1-irogers@google.com
(cherry picked from commit 9683e577)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>