net: icmpv6: Simplify the flow at net_icmpv6_get/set_xxx() calls

Instead of reading or writing different icmpv6 header's individual
variables, better to read or write whole struct at a time. This
minimizes the calls to net_frag_read() or net_frag_write().
changes also removed slow and fast paths. Changes should optimize
the total flow.

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