bpf: Fix build without CONFIG_STACKTRACE

Without CONFIG_STACKTRACE stack_trace_save_tsk() is not defined. Let
get_callchain_entry_for_task() to always return NULL in such cases.

Fixes: fa28dcb8 ("bpf: Introduce helper bpf_get_task_stack()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200703024537.79971-1-songliubraving@fb.com

kernel/bpf/stackmap.c

@@ -351,6 +351,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
 static struct perf_callchain_entry *
 get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
 {
+#ifdef CONFIG_STACKTRACE
 	struct perf_callchain_entry *entry;
 	int rctx;
 
@@ -380,6 +381,9 @@ get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
 	put_callchain_entry(rctx);
 
 	return entry;
+#else /* CONFIG_STACKTRACE */
+	return NULL;
+#endif
 }
 
 BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,