net: context: Fix setting of appdata ptr and length

When IPv6 header contains extra header (e.g. HBH), extra header length
was not part of app data length calculation. Result applications get
invalid total length. Also simplified set_appdata_values().

Change-Id: I07438d62dfd0f9abc1452484deb8b5eacdc37cf1
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>