ospf6d: replace OSPF6_LSA_HEADER_END macro

ospf6d: replace OSPF6_LSA_HEADER_END macro

Replacing the macro with an inline function allows the compiler to
check the parameter type.

Use the replacement function consistently to reduce the number of
open coded pointer cast plus offset calculations.

use tools/indent.py to reformat all occurences of its use.

Signed-off-by: Andrew Cooks <acooks.at.bda@gmail.com>