netfilter: nf_tables: Unconditionally allocate nft_obj_filter

[ Upstream commit 4279cc60 ]

Prep work for moving the filter into struct netlink_callback's scratch
area.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: bd662c42 ("netfilter: nf_tables: Add locking for NFT_MSG_GETOBJ_RESET requests")
Signed-off-by: Sasha Levin <sashal@kernel.org>