zebra: clear host bits of ND RA prefix option

zebra: clear host bits of ND RA prefix option

RFC4861 4.6.2. Prefix Information

Prefix         An IP address or a prefix of an IP address.  The
               Prefix Length field contains the number of valid
               leading bits in the prefix.  The bits in the prefix
               after the prefix length are reserved and MUST be
               initialized to zero by the sender and ignored by
               the receiver.

* rtadv.c
  * ipv6_nd_prefix(): add missing call to apply_mask_ipv6()
  * no_ipv6_nd_prefix(): idem