xen/xsm: Improve alloc/free of evtchn buckets

xen/xsm: Improve alloc/free of evtchn buckets

Currently, flask_alloc_security_evtchn() is called in loops of
64 (EVTCHNS_PER_BUCKET), which for non-dummy implementations is a function
pointer call even in the no-op case.  The non no-op case only sets a single
constant, and doesn't actually fail.

Spectre v2 protections has made function pointer calls far more expensive, and
64 back-to-back calls is a waste.  Rework the APIs to pass the size of the
bucket instead, and call them once.

No practical change, but {alloc,free}_evtchn_bucket() should be rather more
efficient now.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com>