net: Modify IPv4/6 packet finalize API's

Current finalize api's takes buf as input parameter and returns
the finalized buf. But if there are any issue while finalizing,
it failed to throw an error.

Change-Id: I6db54b7453eec41a8051fab50d5c0dc937debd54
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>