[roll] Roll fuchsia [netstack3] Remove incorrect serialize_new_buf() impl in ForwardedPacket

serialize_new_buf() implementation in ForwardedPacket was incorrect: it
was not serializing IP header. Remove it and replace with
unimplemented!().

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1143134
Original-Revision: 692e6899fa96b24590f48baec6e3eed01ca387b9
GitOrigin-RevId: 1cf1789403813eafaf5256254b663b0c8b8b2d72
Change-Id: I4b6c7490270d846dfe503e96d26e9c881a2d063a
diff --git a/stem b/stem
index 0477c7a..5b6728d 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="1195ec8dbbecc73948ed789d1d3a8e094dff9143"/>
+             revision="692e6899fa96b24590f48baec6e3eed01ca387b9"/>
   </projects>
   <hooks>
     <hook name="install-environment"