tests: fix prog_info initialization in bpf-obj_get_info_by_fd-prog*

tests: fix prog_info initialization in bpf-obj_get_info_by_fd-prog*

The sys_bpf(BPF_OBJ_GET_INFO_BY_FD, &bpf_prog_get_info_attr, ...) is
called in a loop.  The bpf_prog_get_info_attr.info object is in size 104
but bpf_prog_get_info_attr.info_len is in size 168.  Hence, if the prog
is jited, the second iteration onwards will have nr_jited_ksyms == 1
which is asking the kernel to fill the
bpf_prog_get_info_attr.info.jited_ksyms and it is NULL.

* tests/bpf-obj_get_info_by_fd.c (main): Clear memory beyond prog_info
at the start of every iteration.

Co-Authored-by: Dmitry V. Levin <ldv@altlinux.org>