netfilter: nft_numgen: enable hashing of one element

The modulus in the hash function was limited to > 1 as initially
there was no sense to create a hashing of just one element.

Nevertheless, there are certain cases specially for load balancing
where this case needs to be addressed.

This patch fixes the following error.

Error: Could not process rule: Numerical result out of range
add rule ip nftlb lb01 dnat to jhash ip saddr mod 1 map { 0: 192.168.0.10 }
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The solution comes to force the hash to 0 when the modulus is 1.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>