bdev/nvme: add type_ctx to nvme_bdev_ns

This gives per-type ns handlers a way to associate
the generic nvme_bdev_ns structure to any type-specific
context.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Id632b41bc3ad842c8ec5e836436bd081e8cad496

Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/475926


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>