sparse: Make IN6_IS_ADDR_MC_LINKLOCAL and IN6_ARE_ADDR_EQUAL pickier.

sparse: Make IN6_IS_ADDR_MC_LINKLOCAL and IN6_ARE_ADDR_EQUAL pickier.

On GNU systems these macros work with arbitrary pointers, but the relevant
standards only require IN6_IS_ADDR_MC_LINKLOCAL to work with in6_addr (and
don't specify IN6_ARE_ADDR_EQUAL at all).  Make the "sparse"
implementations correspondingly pickier so that we catch any introduced
problems more quickly.

CC: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Aaron Conole <aconole@redhat.com>