zebra: set zd_is_update in 1 spot

zebra: set zd_is_update in 1 spot

The ctx->zd_is_update is being set in various
spots based upon the same value that we are
passing into dplane_ctx_ns_init.  Let's just
consolidate all this into the dplane_ctx_ns_init
so that the zd_is_udpate value is set at the
same time that we increment the sequence numbers
to use.

As a note for future me's reading this.  The sequence
number choosen for the seq number passed to the
kernel is that each context gets a copy of the
appropriate nlsock to use.  Since it's a copy
at a point in time, we know we have a unique sequence
number value.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
(cherry picked from commit e3ee55d4bde5de9d39a91b53b30f67a6e45bbdb8)