evtchn: avoid NULL derefs

evtchn: avoid NULL derefs

Commit fbbd5009e6 ("evtchn: refactor low-level event channel port ops")
added a de-reference of the struct evtchn pointer for a port without
first making sure the bucket pointer is non-NULL. This de-reference is
actually entirely unnecessary, as all relevant callers (beyond the
problematic do_poll()) already hold the port number in their hands, and
the actual leaf functions need nothing else.

For FIFO event channels there's a second problem in that the ordering
of reads and updates to ->num_evtchns and ->event_array[] was so far
undefined (the read side isn't always holding the domain's event lock).
Add respective barriers.

This is XSA-221.

Reported-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>