net: dns: Check existing DNS servers before reconfigure

In dns_resolve_reconfigure() check if the DNS servers already exist
before cancel all ongoing queries. This will solve an issue with
getaddrinfo() returning DNS_EAI_CANCELED when receiving a retransmitted
DHCP offer and when receiving a IPv6 Router Advertisement.

Signed-off-by: Stig Bjørlykke <stig.bjorlykke@nordicsemi.no>