update_engine: Fix memory leak on new ext2_filesystem code.

ext2fs_get_pathname() allocates new memory to store the pathname and we
are supposed to free it. This patch adds the missing call to
ext2fs_free_mem().

BUG=None
TEST=`FEATURES=test USE="clang asan" VERBOSE=1 GTEST_FILTER="Ext2*" emerge-link update_engine` works now and didn't work before

Change-Id: I5244e7f9721875590c3b2455d469866d28c70c09
Reviewed-on: https://chromium-review.googlesource.com/277277
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/payload_generator/ext2_filesystem.cc b/payload_generator/ext2_filesystem.cc
index 0467b80..9670e39 100644
--- a/payload_generator/ext2_filesystem.cc
+++ b/payload_generator/ext2_filesystem.cc
@@ -250,6 +250,7 @@
       files->push_back(inodes[dir_ino]);
       used_inodes.insert(dir_ino);
     }
+    ext2fs_free_mem(&dir_name);
 
     error = ext2fs_dir_iterate2(
         filsys_, dir_ino, 0, nullptr /* block_buf */,