netfilter: nf_tables: do not leave chain stats enabled on error

Error might occur later in the nf_tables_addchain() codepath, enable
static key only after transaction has been created.

Fixes: 9f08ea84

 ("netfilter: nf_tables: keep chain counters away from hot path")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>