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: bc235cdb423a ("bpf: Prevent deadlock from recursive bpf_task_storage_[get|delete]")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Wei Yongjun <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/bpf_task_storage.c

···
 
 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)
 {