fastboot: Cap max size sent to libsparse

This is required for large (>INT_MAX) sparse limit reported by
the target.

Also, patch up return chains of "int" that need to deal with sizes
bigger than 2GB as well as return negative error codes.

Test: -S works with large max-download-size
Test: Flash 3GB system.img with max-download-size 2.5GB

Bug: 36810152
Change-Id: I562a50eabd706bd5b97c71a1aef07c1ffd1a2e5c
diff --git a/fastboot/engine.cpp b/fastboot/engine.cpp
index 728dcb8..9f7d226 100644
--- a/fastboot/engine.cpp
+++ b/fastboot/engine.cpp
@@ -328,11 +328,11 @@
     queue_action(OP_WAIT_FOR_DISCONNECT, "");
 }
 
-int fb_execute_queue(Transport* transport)
+int64_t fb_execute_queue(Transport* transport)
 {
     Action *a;
     char resp[FB_RESPONSE_SZ+1];
-    int status = 0;
+    int64_t status = 0;
 
     a = action_list;
     if (!a)