net: dns: Fix timeout calculation with DNS retransmissions

With recently introduced DNS retransmission mechanism, a certain bug
could occur when calculating query timeout.

If the time until the final DNS timeout (as indicated by
CONFIG_NET_SOCKETS_DNS_TIMEOUT) was less than 1 millisecond, the actual
millisecond timeout value was rounded down, resulting in 0 ms timeout.
This in order was interpreted as invalid argument by dns_get_addr_info()
function, so in result, instead of reporting query timeout, the function
reported invalid argument error.

Fix this by rounding the millisecond timeout up, instead of down, so
that in any case, if the final timeout is not due, we always provide
non-zero timeout to dns_get_addr_info().

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