bpf: Make symbol 'bpf_task_storage_busy' static

The sparse tool complains as follows:

kernel/bpf/bpf_task_storage.c:23:1: warning:
 symbol '__pcpu_scope_bpf_task_storage_busy' was not declared. Should it be static?

This symbol is not used outside of bpf_task_storage.c, so this
commit marks it static.

Fixes: bc235cdb ("bpf: Prevent deadlock from recursive bpf_task_storage_[get|delete]")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210311131505.1901509-1-weiyongjun1@huawei.com

kernel/bpf/bpf_task_storage.c

@@ -20,7 +20,7 @@
 
 DEFINE_BPF_STORAGE_CACHE(task_cache);
 
-DEFINE_PER_CPU(int, bpf_task_storage_busy);
+static DEFINE_PER_CPU(int, bpf_task_storage_busy);
 
 static void bpf_task_storage_lock(void)
 {