selftests/bpf: Use BPF_PROG2 for some fentry programs without struct arguments

Use BPF_PROG2 instead of BPF_PROG for programs in progs/timer.c
to test BPF_PROG2 for cases without struct arguments.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220831152718.2081091-1-yhs@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>