netfilter: nf_tables: use the right index from flowtable error path

Use the right loop index, not the number of devices in the array that we
need to remove, the following message uncovered the problem:

[ 5437.044119] hook not found, pf 5 num 0
[ 5437.044140] WARNING: CPU: 2 PID: 24983 at net/netfilter/core.c:376 __nf_unregister_net_hook+0x250/0x280

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