netfilter: nft_connlimit: memleak if nf_ct_netns_get() fails

Check if nf_ct_netns_get() fails then release the limit object
previously allocated via kmalloc().

Fixes: 37f319f3 ("netfilter: nft_connlimit: move stateful fields out of expression data")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>