We may have pending skbs in the receive queue when the sk is being
destroyed; add a destructor to purge the queue.

MCTP doesn't use the error queue, so only the receive_queue is purged.

Fixes: 833ef3b9 ("mctp: Populate socket implementation")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://lore.kernel.org/r/20230126064551.464468-1-jk@codeconstruct.com.auSigned-off-by: Jakub Kicinski <kuba@kernel.org>