netfilter: nf_tables: use the correct get/put helpers

Switch to be16/32 and u16/32 respectively.  No code changes here,
the functions do the same thing, this is just for sparse checkers' sake.

objdiff shows no changes.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>