netfilter: nftables: add helper function to set the base sequence number

[ 802b805162a1b7d8391c40ac8a878e9e63287aff ]

This patch adds a helper function to calculate the base sequence number
field that is stored in the nfnetlink header. Use the helper function
whenever possible.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>