bpftool: Fix control flow graph segfault during edge creation

If the last instruction of a control flow graph building block is a
BPF_CALL, an incorrect edge with e->dst set to NULL is created and
results in a segfault during graph output.

Ensure that BPF_CALL as last instruction of a building block is handled
correctly and only generates a single edge unlike actual BPF_JUMP*
instructions.

Signed-off-by: Christoph Werle <christoph.werle@longjmp.de>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Quentin Monnet <qmo@kernel.org>
Reviewed-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20250108220937.1470029-1-christoph.werle@longjmp.de

Author: troglodyt

tools/bpf/bpftool/cfg.c

@@ -302,6 +302,7 @@ static bool func_add_bb_edges(struct func_node *func)
 
 		insn = bb->tail;
 		if (!is_jmp_insn(insn->code) ||
+		    BPF_OP(insn->code) == BPF_CALL ||
 		    BPF_OP(insn->code) == BPF_EXIT) {
 			e->dst = bb_next(bb);
 			e->flags |= EDGE_FLAG_FALLTHROUGH;