audit: explicitly check audit_context->context enum value

Be explicit in checking the struct audit_context "context" member enum
value rather than assuming the order of context enum values.

Fixes: 12c5e81d ("audit: prepare audit_context for use in calling contexts beyond syscalls")
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>