net: lib: dns: Fix DNS dispatcher double net buf unref

The `dns_data` buffer, allocated by the DNS dispatcher was dereferenced
twice - once in registered DNS handler, second time in the dispatcher
itself.

Since the buffer was allocated by the dispatcher, and it's not really
guaranteed that the buffer will be freed in the registered handler (this
depends on the processing outcome, the function may return early w/o
freeing the net buf in case of errors), it makes most sense for the
dispatcher to keep ownership of the buffer. Hence, the registered
handlers will no longer release the buffer provided in any case, and the
dispatcher will free it on exit.

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