nft_genmask_cur has already done left-shift operator on the gencursor,
so there's no need to do left-shift operator on it again.

Fixes: ea4bd995 ("netfilter: nf_tables: add transaction helper functions")
Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>