mnl: fix inclusion of last rule in batch page

mnl: fix inclusion of last rule in batch page

This patch fixes the inclusion of the last rule that didn't fit
into a batch page.

When using sets this has manifested with the -EBUSY error when deleting
the table (it was still containing unused sets after the flush).

The following command line works fine here:

nft -f test ; nft flush table filter ; nft delete chain filter output; nft delete table filter

Tested using this kernel patch: http://patchwork.ozlabs.org/patch/314143/

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