xsm/flask: buffer AVC messages for output

xsm/flask: buffer AVC messages for output

When multiple CPUs hit an AVC audit message, the resulting output in
the ring buffer and serial console is garbled due to the audit process
using many separate printk invocations for each message. Change the
AVC audit process to use a temporary buffer and output the contents
once the entire audit message is complete.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Keir Fraser <keir@xen.org>