netfilter: nft_last: avoid possible false sharing

Use the idiom described in:

https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance



Moreover, prevent a compiler optimization.

Fixes: 836382dc ("netfilter: nf_tables: add last expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>