netfilter: nf_tables: remove unused variable

Variable 'ext' is being assigned but are never used hence they are
unused and can be removed.

Cleans up clang warnings:
net/netfilter/nf_tables_api.c:4032:28: warning: variable ‘ext’ set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>