evtchn/FIFO: add 2nd smp_rmb() to evtchn_fifo_word_from_port()

evtchn/FIFO: add 2nd smp_rmb() to evtchn_fifo_word_from_port()

Besides with add_page_to_event_array() the function also needs to
synchronize with evtchn_fifo_init_control() setting both d->evtchn_fifo
and (subsequently) d->evtchn_port_ops.

This is XSA-359 / CVE-2020-29571.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
master commit: dc8b01affd7f6f36d34c3854f51df0847df3ec0e
master date: 2020-12-15 13:42:51 +0100