selftests/bpf: Ignore .llvm.<hash> suffix in kallsyms_find()

selftests/bpf: Ignore .llvm.<hash> suffix in kallsyms_find()

I hit the following failure when running selftests with
internal backported upstream kernel:
  test_ksyms:PASS:kallsyms_fopen 0 nsec
  test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found
  #123     ksyms:FAIL

In /proc/kallsyms, we have
  $ cat /proc/kallsyms | grep bpf_link_fops
  ffffffff829f0cb0 d bpf_link_fops.llvm.12608678492448798416
The CONFIG_LTO_CLANG_THIN is enabled in the kernel which is responsible
for bpf_link_fops.llvm.12608678492448798416 symbol name.

In prog_tests/ksyms.c we have
  kallsyms_find("bpf_link_fops", &link_fops_addr)
and kallsyms_find() compares "bpf_link_fops" with symbols
in /proc/kallsyms in order to find the entry. With
bpf_link_fops.llvm.<hash> in /proc/kallsyms, the kallsyms_find()
failed.

To fix the issue, in kallsyms_find(), if a symbol has suffix
.llvm.<hash>, that suffix will be ignored for comparison.
This fixed the test failure.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240604180034.1356016-1-yonghong.song@linux.dev