zebra: Remove `struct nlsock` from dataplane information and use `int fd`

zebra: Remove `struct nlsock` from dataplane information and use `int fd`

Store the fd that corresponds to the appropriate `struct nlsock` and pass
that around in the dplane context instead of the pointer to the nlsock.
Modify the kernel_netlink.c code to store in a hash the `struct nlsock`
with the socket fd as the key.

Why do this?  The dataplane context is used to pass around the `struct nlsock`
but the zebra code has a bug where the received buffer for kernel netlink
messages from the kernel is not big enough.  So we need to dynamically
grow the receive buffer per socket, instead of having a non-dynamic buffer
that we read into.  By passing around the fd we can look up the `struct nlsock`
that will soon have the associated buffer and not have to worry about `const`
issues that will arise.

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