evtchn/FIFO: re-order and synchronize (with) map_control_block()

evtchn/FIFO: re-order and synchronize (with) map_control_block()

For evtchn_fifo_set_pending()'s check of the control block having been
set to be effective, ordering of respective reads and writes needs to be
ensured: The control block pointer needs to be recorded strictly after
the setting of all the queue heads, and it needs checking strictly
before any uses of them (this latter aspect was already guaranteed).

This is XSA-358 / CVE-2020-29570.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>