bpf: devmap, use cond_resched instead of cpu_relax

Be a bit more friendly about waiting for flush bits to complete.
Replace the cpu_relax() with a cond_resched().

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/devmap.c

@@ -159,7 +159,7 @@ static void dev_map_free(struct bpf_map *map)
 		unsigned long *bitmap = per_cpu_ptr(dtab->flush_needed, cpu);
 
 		while (!bitmap_empty(bitmap, dtab->map.max_entries))
-			cpu_relax();
+			cond_resched();
 	}
 
 	for (i = 0; i < dtab->map.max_entries; i++) {