selftests/bpf: Test kernel module ksym externs

Add per-CPU variable to bpf_testmod.ko and use those from new selftest to
validate it works end-to-end.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20210112075520.4103414-8-andrii@kernel.org

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -3,6 +3,7 @@
 #include <linux/error-injection.h>
 #include <linux/init.h>
 #include <linux/module.h>
+#include <linux/percpu-defs.h>
 #include <linux/sysfs.h>
 #include <linux/tracepoint.h>
 #include "bpf_testmod.h"
@@ -10,6 +11,8 @@
 #define CREATE_TRACE_POINTS
 #include "bpf_testmod-events.h"
 
+DEFINE_PER_CPU(int, bpf_testmod_ksym_percpu) = 123;
+
 noinline ssize_t
 bpf_testmod_test_read(struct file *file, struct kobject *kobj,
 		      struct bin_attribute *bin_attr,