audit: export audit_log_task_info

At the suggestion of eparis@redhat.com, move this chunk of task
logging from audit_log_exit to audit_log_task_info and export this
function so it's usuable elsewhere in the kernel.

This patch is against
git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity#next-ima-appraisal



Changelog v2:
 - add empty audit_log_task_info if CONFIG_AUDITSYSCALL isn't set.

Changelog v1:
 - Initial post.

Signed-off-by: Peter Moody <pmoody@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>