bpf: Use prog->jited_len in bpf_prog_ksym_set_addr()

Using prog->jited_len is simpler and more accurate than current
estimation (header + header->size).

Also, fix missing prog->jited_len with multi function program. This hasn't
been a real issue before this.

Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220204185742.271030-5-song@kernel.org