Replace dead MAX_AUDIT_MESSAGE_LENGTH comment link
Replace dead MAX_AUDIT_MESSAGE_LENGTH comment link

Replaces the link that is currently stale for `MAX_AUDIT_MESSAGE_LENGTH` with one tied to a specific linux kernel tag (so the line number remains constant/correct) from the official torvalds linux repo on Github.
slackhq/go-audit,slackhq/go-audit,slackhq/go-audit