ar: handle partial writes from archive_write_data

libarchive may limit a single archive_write_data call to handling
0x7fffffff bytes. Add a loop to handle partial writes.

Reviewed by:	kib, jhb
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D11715

usr.bin/ar/write.c

@@ -586,10 +586,17 @@ prefault_buffer(const char *buf, size_t s)
 static void
 write_data(struct bsdar *bsdar, struct archive *a, const void *buf, size_t s)
 {
+	ssize_t written;
+
 	prefault_buffer(buf, s);
-	if (archive_write_data(a, buf, s) != (ssize_t)s)
-		bsdar_errc(bsdar, EX_SOFTWARE, 0, "%s",
-		    archive_error_string(a));
+	while (s > 0) {
+		written = archive_write_data(a, buf, s);
+		if (written < 0)
+			bsdar_errc(bsdar, EX_SOFTWARE, 0, "%s",
+			    archive_error_string(a));
+		buf = (const char *)buf + written;
+		s -= written;
+	}
 }
 
 /*