netfilter: nf_tables: Introduce nft_set_dump_ctx_init()

This is a wrapper around nft_ctx_init() for use in
nf_tables_getsetelem() and a resetting equivalent introduced later.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>