net: sockets: getaddrinfo: Fix possible crash when callback is delayed

In case system workqueue processing is delayed for any reason, and
resolver callback is executed after getaddrinfo() call already timed
out, the system would crash as the callback makes use of the user data
allocated on the stack within getaddrinfo() function.

Prevent that, by cancelling the DNS request explicitly from the
getaddrinfo() context, therefore preventing the resolver callback
from being executed after the getaddrinfo() call ends.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>