vmlinux.c uses the `CHECK` calls even though the use of ASSERT_ series
of macros is preferred in the bpf selftests.

This patch replaces all `CHECK` calls for equivalent `ASSERT_`
macro calls.
Signed-off-by: Yuran Pereira <yuran.pereira@hotmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/GV1PR10MB6563ED1023A2A3AEF30BDA5DE8BBA@GV1PR10MB6563.EURPRD10.PROD.OUTLOOK.COM