netfilter: xt_hashlimit: fix race that results in duplicated entries

Two packets may race to create the same entry in the hashtable,
double check if this packet lost race. This double checking only
happens in the path of the packet that creates the hashtable for
first time.

Note that, with this patch, no packet drops occur if the race happens.

Reported-by: Feng Gao <gfree.wind@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>