iptables: nft: fix memory leaks in nft_fini

iptables: nft: fix memory leaks in nft_fini

Those errors are shown with valgrind tool:

valgrind --leak-check=full xtables -A INPUT -i eth0 -p tcp --dport 80

==12554== 40 bytes in 1 blocks are still reachable in loss record 1 of 10
==12554==    at 0x4C2935B: malloc (vg_replace_malloc.c:270)
==12554==    by 0x574D755: mnl_nlmsg_batch_start (nlmsg.c:447)
==12554==    by 0x416520: nft_action (nft.c:2281)
==12554==    by 0x41355E: xtables_main (xtables-standalone.c:75)
==12554==    by 0x5B87994: (below main) (libc-start.c:260)

==12554== 135,168 bytes in 1 blocks are still reachable in loss record 9 of 10
==12554==    at 0x4C2935B: malloc (vg_replace_malloc.c:270)
==12554==    by 0x415A24: mnl_nft_batch_alloc (nft.c:102)
==12554==    by 0x416520: nft_action (nft.c:2281)
==12554==    by 0x41355E: xtables_main (xtables-standalone.c:75)
==12554==    by 0x5B87994: (below main) (libc-start.c:260)

These objects are allocated from nft_init but they were not released
appropriately in the exit path.

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>