bpf: Factor out visit_func_call_insn() in check_cfg()

During verifier check_cfg(), all instructions are
visited to ensure verifier can handle program control flows.
This patch factored out function visit_func_call_insn()
so it can be reused in later patch to visit callback function
calls. There is no functionality change for this patch.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226204920.3884136-1-yhs@fb.com