When copying file data to the archive, don't write more

than was read.  This seems to have only affected the shar
writer, since other formats proactively truncate output
to the originally-advertised size.

PR:		bin/131244
MFC after:	7 days

usr.bin/tar/write.c

@@ -972,7 +972,7 @@ write_file_data(struct bsdtar *bsdtar, struct archive *a,
 		siginfo_printinfo(bsdtar, progress);
 
 		bytes_written = archive_write_data(a, bsdtar->buff,
-		    FILEDATABUFLEN);
+		    bytes_read);
 		if (bytes_written < 0) {
 			/* Write failed; this is bad */
 			bsdtar_warnc(bsdtar, 0, "%s", archive_error_string(a));