loader: tftp: Copy the first block into the cache

tftp_open reads the first block so copy it in the cached data.
If we have more than one block (i.e. we called tftp_read before
tftp_preload) simply just reset the transfer.

Reported by:	mmel
Reviewed by:	mmel, tsoome
MFC after:	2 weeks
Sponsored by:	Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D33697

stand/libsa/tftp.c

@@ -680,6 +680,13 @@ tftp_preload(struct open_file *f)
 	start = getsecs();
 	printf("Preloading %s ", tftpfile->path);
 #endif
+	if (tftpfile->currblock == 1)
+		bcopy(tftpfile->tftp_hdr->th_data,
+		    cache,
+		    tftpfile->validsize);
+	else
+		tftpfile->currblock = 0;
+
 	while (tftpfile->islastblock == 0) {
 		twiddle(32);
 		rc = tftp_getnextblock(tftpfile);