netfilter: nft_set_pipapo: Actually validate intervals in fields after the first one

netfilter: nft_set_pipapo: Actually validate intervals in fields after the first one

Embarrassingly, nft_pipapo_insert() checked for interval validity in
the first field only.

The start_p and end_p pointers were reset to key data from the first
field at every iteration of the loop which was supposed to go over
the set fields.

Fixes: 14357d3fdd6c ("nf_tables: Add set type for arbitrary concatenation of ranges")
Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>