ipv6: ioam: Prerequisite patch for ioam6_iptunnel

This prerequisite patch provides some minor edits (alignments, renames) and a
minor modification inside a function to facilitate the next patch by using
existing nla_* functions.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: David S. Miller <davem@davemloft.net>