udp: Prepare udp.c to be shared with vhost-user

Export udp_payload_t, udp_update_hdr4(), udp_update_hdr6() and
udp_sock_errs().

Rename udp_listen_sock_handler() to udp_buf_listen_sock_handler() and
udp_reply_sock_handler to udp_buf_reply_sock_handler().

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

udp_internal.h

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright (c) 2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
+#ifndef UDP_INTERNAL_H
+#define UDP_INTERNAL_H
+
+#include "tap.h" /* needed by udp_meta_t */
+
+#define UDP_MAX_FRAMES		32  /* max # of frames to receive at once */
+
+/**
+ * struct udp_payload_t - UDP header and data for inbound messages
+ * @uh:		UDP header
+ * @data:	UDP data
+ */
+struct udp_payload_t {
+	struct udphdr uh;
+	char data[USHRT_MAX - sizeof(struct udphdr)];
+#ifdef __AVX2__
+} __attribute__ ((packed, aligned(32)));
+#else
+} __attribute__ ((packed, aligned(__alignof__(unsigned int))));
+#endif
+
+size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
+		       const struct flowside *toside, size_t dlen,
+		       bool no_udp_csum);
+size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
+                       const struct flowside *toside, size_t dlen,
+		       bool no_udp_csum);
+int udp_sock_errs(const struct ctx *c, int s, uint32_t events);
+#endif /* UDP_INTERNAL_H */