inlet/flow: enforce compatibility with Linux 4.19

The two last fields of `sk_reuseport_md` were added in Linux 5.14. We
don't use them, so it shouldn't matter. I remove them from `vmlinux.h`
to ensure compatibility. Also, adding
`__attribute__((preserve_access_index))` should make the program more
portable (BPF CO-RE).

inlet/flow/input/udp/vmlinux.h

@@ -49,21 +49,7 @@ struct sk_reuseport_md {
     __u32 ip_protocol;	/* IP protocol. e.g. IPPROTO_TCP, IPPROTO_UDP */
     __u32 bind_inany;	/* Is sock bound to an INANY address? */
     __u32 hash;		/* A hash of the packet 4 tuples */
-    /* When reuse->migrating_sk is NULL, it is selecting a sk for the
-     * new incoming connection request (e.g. selecting a listen sk for
-     * the received SYN in the TCP case).  reuse->sk is one of the sk
-     * in the reuseport group. The bpf prog can use reuse->sk to learn
-     * the local listening ip/port without looking into the skb.
-     *
-     * When reuse->migrating_sk is not NULL, reuse->sk is closed and
-     * reuse->migrating_sk is the socket that needs to be migrated
-     * to another listening socket.  migrating_sk could be a fullsock
-     * sk that is fully established or a reqsk that is in-the-middle
-     * of 3-way handshake.
-     */
-    __bpf_md_ptr(struct bpf_sock *, sk);
-    __bpf_md_ptr(struct bpf_sock *, migrating_sk);
-};
+} __attribute__((preserve_access_index));
 
 /*
  * bpf_map_lookup_elem