AUDIT: Assign serial number to non-syscall messages

AUDIT: Assign serial number to non-syscall messages

Move audit_serial() into audit.c and use it to generate serial numbers
on messages even when there is no audit context from syscall auditing.
This allows us to disambiguate audit records when more than one is
generated in the same millisecond.

Based on a patch by Steve Grubb after he observed the problem.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>