selftests/bpf: Convert CHECK macros to ASSERT_* macros in bpf_iter

selftests/bpf: Convert CHECK macros to ASSERT_* macros in bpf_iter

As it was pointed out by Yonghong Song [1], in the bpf selftests the use
of the ASSERT_* series of macros is preferred over the CHECK macro.
This patch replaces all CHECK calls in bpf_iter with the appropriate
ASSERT_* macros.

[1] https://lore.kernel.org/lkml/0a142924-633c-44e6-9a92-2dc019656bf2@linux.dev

Suggested-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Yuran Pereira <yuran.pereira@hotmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/DB3PR10MB6835E9C8DFCA226DD6FEF914E8A3A@DB3PR10MB6835.EURPRD10.PROD.OUTLOOK.COM
Signed-off-by: Alexei Starovoitov <ast@kernel.org>