netfilter: nf_tables: fix memory leak if expr init fails

If expr init fails then we need to free it.

So when the user add a nft rule as follows:

  # nft add rule filter input tcp dport 22 flow table ssh \
    { ip saddr limit rate 0/second }

memory leak will happen.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>