src: remove global nftnl_batch structure in mnl layer

src: remove global nftnl_batch structure in mnl layer

The underlying mnl layer uses a global nftnl_batch structure. Instead,
pass pointer as parameter to the functions that need this. The netlink
layer stores a reference to this structure in struct netlink_ctx.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>