src: allow to use integer type header fields via typeof set declaration

src: allow to use integer type header fields via typeof set declaration

Header fields such as udp length cannot be used in concatenations because
it is using the generic integer_type:

 test.nft:3:10-19: Error: can not use variable sized data types (integer) in concat expressions
                typeof udp length . @th,32,32
                       ^^^^^^^^^^~~~~~~~~~~~~

This patch slightly extends ("src: allow to use typeof of raw expressions in
set declaration") to set on NFTNL_UDATA_SET_KEY_PAYLOAD_LEN in userdata if
TYPE_INTEGER is used.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>