xdp: Make __mem_id_disconnect static

Fix sparse warning:

net/core/xdp.c:88:6: warning:
 symbol '__mem_id_disconnect' was not declared. Should it be static?
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NJesper Dangaard Brouer <brouer@redhat.com>
Acked-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

net/core/xdp.c

@@ -85,7 +85,7 @@ static void __xdp_mem_allocator_rcu_free(struct rcu_head *rcu)
 	kfree(xa);
 }
 
-bool __mem_id_disconnect(int id, bool force)
+static bool __mem_id_disconnect(int id, bool force)
 {
 	struct xdp_mem_allocator *xa;
 	bool safe_to_remove = true;