BUG/MINOR: dns: fix ring offset calculation in dns_resolve_send()

BUG/MINOR: dns: fix ring offset calculation in dns_resolve_send()

With 737d10f ("BUG/MEDIUM: dns: ensure ring offset is properly reajusted
to head") relative offset calculation was fixed in dns_session_io_handler()
and dns_process_req() functions.

But if we compare with the changes performed in the patch that introduced
the bug: d9c7188 ("MEDIUM: ring: make the offset relative to the head/tail
instead of absolute"), we can see that dns_resolve_send() is missing from
the patch.

Applying both 737d10f + ("BUG/MINOR: dns: fix ring offset calculation on
first read") to dns_resolve_send() function.
With this last commit, we should be back at pre d9c7188 behavior.

No backport needed.