netfilter: nf_tables: add function to create set stateful expressions

[ Upstream commit a8fe4154 ]

Add a helper function to allocate and initialize the stateful expressions
that are defined in a set.

This patch allows to reuse this code from the set update path, to check
that type of the update matches the existing set in the kernel.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: f6594c37 ("netfilter: nf_tables: perform type checking for existing sets")
Signed-off-by: Sasha Levin <sashal@kernel.org>