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/fastboot.cpp b/fastboot/fastboot.cpp
index 704dc43..cb8e5c0 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -78,6 +78,10 @@
 static const char* cmdline = nullptr;
 static unsigned short vendor_id = 0;
 static int long_listing = 0;
+// Don't resparse files in too-big chunks.
+// libsparse will support INT_MAX, but this results in large allocations, so
+// let's keep it at 1GB to avoid memory pressure on the host.
+static constexpr int64_t RESPARSE_LIMIT = 1 * 1024 * 1024 * 1024;
 static int64_t sparse_limit = -1;
 static int64_t target_sparse_limit = -1;
 
@@ -789,7 +793,7 @@
     }
 
     if (size > limit) {
-        return limit;
+        return std::min(limit, RESPARSE_LIMIT);
     }
 
     return 0;