zebra: Refactor nexthop group creation code to use allocated memory

zebra: Refactor nexthop group creation code to use allocated memory

Simplify the code for nexthop hash entry creation. I made nexthop
hash entry creation expect the nexthop group and depends to always
be allocated before lookup. Before, it was only allocated if it had
dependencies. I think it makes the code a bit more readable to go
ahead an allocate even for single nexthops as well.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>