MINOR: quic: fix return of quic_dgram_read

It is expected that quic_dgram_read() returns the total number of bytes
read. Fix the return value when the read has been successful. This bug
has no impact as in the end the return value is not checked by the
caller.

src/xprt_quic.c

@@ -5371,7 +5371,7 @@ static ssize_t quic_dgram_read(struct buffer *buf, size_t len, void *owner,
 	if (dgram_ctx.qc)
 		dgram_ctx.qc->rx.bytes += len;
 
-	return pos - (unsigned char *)buf;
+	return pos - (unsigned char *)b_head(buf);
 
  err:
 	return -1;