audit: Make audit_filter_syscall() return void

No invoker uses the return value of audit_filter_syscall().
So make it return void, and amend the comment of
audit_filter_syscall().

Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
[PM: removed the changelog from the description]
Signed-off-by: Paul Moore <paul@paul-moore.com>