netfilter: nf_tables: remove unused variable in nf_tables_dump_set()

The nfmsg variable is not used (except in sizeof operator which does
not care about its value) between the first and second time it is
assigned the value.  Furthermore, nlmsg_data has no side effects, so
the assignment can be safely removed.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>