Allow to shrink ext2 filesystems.

There was a function a long time ago that didn't handle shrinking an
ext2 filesystem when computing the diff of the blocks that were not
part of a file nor metadata (mostly empty space blocks). This is now
handled in a different way, which is not related to how the filesystems
are parsed in the ABGenerator.

This patch removes the check that prevents the delta generator from
generating those payloads and fixes the inplace generator for partitions
that shrink in size.

Bug: 28846535
TEST=Generated a payload to a smaller ext2 system image, both for
minor version 1 and 2+.

Change-Id: Ide408c48b0032dbe478c8e348c40e1a0b5665ea8
diff --git a/payload_generator/inplace_generator.h b/payload_generator/inplace_generator.h
index 4839824..48a1fac 100644
--- a/payload_generator/inplace_generator.h
+++ b/payload_generator/inplace_generator.h
@@ -212,6 +212,7 @@
   // On success, stores the new operations in |aops| in the right order and
   // returns true.
   static bool ResolveReadAfterWriteDependencies(
+      const PartitionConfig& old_part,
       const PartitionConfig& new_part,
       uint64_t partition_size,
       size_t block_size,