[ipv6] Fix definition of IN6_IS_ADDR_LINKLOCAL()

Fix an erroneous htonl() in the definition of IN6_IS_ADDR_LINKLOCAL(),
and add self-tests for the IN6_IS_ADDR_xxx() family of macros.

Reported-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>