dnsdist: Fix handling of large XSK frames

dnsdist: Fix handling of large XSK frames

There was a bug in the way we were computing the remaining capacity
of a XSK frame, because we forgot to account for the network headers.
This caused some XSK responses to be discarded by the kernel (`tx_invalid_descs`)
because there was not enough space left in the frame (less than
`XDP_PACKET_HEADROOM`).

Thanks to `ednaq` for reporting this via ou YesWeHack program.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>