MINOR: quic: Get rid of a struct buffer in quic_lstnr_dgram_read()

This is to be sure xprt functions do not manipulate the buffer struct
passed as parameter to quic_lstnr_dgram_read() from low level datagram
I/O callback in quic_sock.c (quic_sock_fd_iocb()).
diff --git a/include/haproxy/xprt_quic.h b/include/haproxy/xprt_quic.h
index ab52808..a24df74 100644
--- a/include/haproxy/xprt_quic.h
+++ b/include/haproxy/xprt_quic.h
@@ -1184,7 +1184,7 @@
 
 void quic_set_tls_alert(struct quic_conn *qc, int alert);
 int quic_set_app_ops(struct quic_conn *qc, const unsigned char *alpn, size_t alpn_len);
-ssize_t quic_lstnr_dgram_read(struct buffer *buf, size_t len, void *owner,
+ssize_t quic_lstnr_dgram_read(unsigned char *buf, size_t len, void *owner,
                               struct sockaddr_storage *saddr);
 
 #endif /* USE_QUIC */
diff --git a/src/quic_sock.c b/src/quic_sock.c
index ee6a2fa..ef94acb 100644
--- a/src/quic_sock.c
+++ b/src/quic_sock.c
@@ -212,7 +212,7 @@
 	} while (0);
 
 	b_add(buf, ret);
-	quic_lstnr_dgram_read(buf, ret, l, &saddr);
+	quic_lstnr_dgram_read((unsigned char *)b_head(buf), ret, l, &saddr);
 	b_del(buf, ret);
  out:
 	MT_LIST_APPEND(&l->rx.rxbuf_list, &rxbuf->mt_list);
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 8a9b4f2..e08a1a4 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -5527,7 +5527,7 @@
  * as owner calling <func> function.
  * Return the number of bytes read if succeeded, -1 if not.
  */
-static ssize_t quic_dgram_read(struct buffer *buf, size_t len, void *owner,
+static ssize_t quic_dgram_read(unsigned char *buf, size_t len, void *owner,
                                struct sockaddr_storage *saddr, qpkt_read_func *func)
 {
 	unsigned char *pos;
@@ -5538,7 +5538,7 @@
 		.owner = owner,
 	};
 
-	pos = (unsigned char *)b_head(buf);
+	pos = buf;
 	end = pos + len;
 	do {
 		int ret;
@@ -5563,13 +5563,13 @@
 	if (dgram_ctx.qc)
 		dgram_ctx.qc->rx.bytes += len;
 
-	return pos - (unsigned char *)b_head(buf);
+	return pos - buf;
 
  err:
 	return -1;
 }
 
-ssize_t quic_lstnr_dgram_read(struct buffer *buf, size_t len, void *owner,
+ssize_t quic_lstnr_dgram_read(unsigned char *buf, size_t len, void *owner,
                               struct sockaddr_storage *saddr)
 {
 	return quic_dgram_read(buf, len, owner, saddr, qc_lstnr_pkt_rcv);