netfilter: nf_tables: Modify nla_memdup's flag to GFP_KERNEL_ACCOUNT

For memory alloc that store user data from nla[NFTA_OBJ_USERDATA],
use GFP_KERNEL_ACCOUNT is more suitable.

Fixes: 33758c89 ("memcg: enable accounting for nft objects")
Signed-off-by: Chen Aotian <chenaotian2@163.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>