net: rpl: Fix setting metadata of forwarding buffer

When DAO message is required to be forwarding to a parent addr,
only frag chain and iface is considered. But for newly created Tx buf
other metadata also required. In this particular case setting
inet type, ip header length. Also update checksum as dst address
is changed.

Change-Id: If0e0a52e943db66be4aeecb6d0e3b9d7c3f04f58
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>