bpf: Fix potential call bpf_link_free() in atomic context

The in_atomic() macro cannot always detect atomic context, in particular,
it cannot know about held spinlocks in non-preemptible kernels. Although,
there is no user call bpf_link_put() with holding spinlock now, be on the
safe side, so we can avoid this in the future.
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200917074453.20621-1-songmuchun@bytedance.com

kernel/bpf/syscall.c

@@ -2345,12 +2345,8 @@ void bpf_link_put(struct bpf_link *link)
 	if (!atomic64_dec_and_test(&link->refcnt))
 		return;
 
-	if (in_atomic()) {
 	INIT_WORK(&link->work, bpf_link_put_deferred);
 	schedule_work(&link->work);
-	} else {
-		bpf_link_free(link);
-	}
 }
 
 static int bpf_link_release(struct inode *inode, struct file *filp)