netfilter: nf_tables: Pass const set to nft_get_set_elem

The function is not supposed to alter the set, passing the pointer as
const is fine and merely requires to adjust signatures of two called
functions as well.

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