evtchn: avoid race in get_xen_consumer()

evtchn: avoid race in get_xen_consumer()

There's no global lock around the updating of this global piece of data.
Make use of cmpxchgptr() to avoid two entities racing with their
updates.

While touching the functionality, mark xen_consumers[] read-mostly (or
else the if() condition could use the result of cmpxchgptr(), writing to
the slot unconditionally).

The use of cmpxchgptr() here points out (by way of clang warning about
it) that its original use of const was slightly wrong. Adjust the
placement, or else undefined behavior of const qualifying a function
type will result.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>