zebra: Fix netlink RTM_NEWNEXTHOP parsing for nested attributes

zebra: Fix netlink RTM_NEWNEXTHOP parsing for nested attributes

With the addition of resillient hashing for nexthops, the
parsing of nexthops requires telling the decoder functions
that there may be nested attributes.  This was found by
code inspection of iproute2/ipnexthop.c when trying to
understand resillient hashing as well as statistics
gathering for nexthops that are / will be in upstream
kernels in the near future.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>