netfilter: nf_tables: fix a typo in nf_tables_getflowtable()

Fix a typo, we should check 'flowtable' instead of 'table'.

Fixes: 3b49e2e9 ("netfilter: nf_tables: add flow table netlink frontend")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>