payload: fix endianess issue in payload_expr_pctx_update()

Use constant_data_ptr() to point to the right memory position on
big endian when exporting data that is stored in a larger variable.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Christophe Leroy <christophe.leroy@c-s.fr>