file: Export receive_fd() to modules

Export receive_fd() so that some modules can use
it to pass file descriptor between processes without
missing any security stuffs.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210831103634.33-4-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>