Define a module version for accept filter modules.

Otherwise accept filters compiled into the kernel do not preempt
preloaded accept filter modules. Then, the preloaded file registers its
accept filter module before the kernel, and the kernel's attempt fails
since duplicate accept filter list entries are not permitted. This
causes the preloaded file's module to be released, since
module_register_init() does a lookup by name, so the preloaded file is
unloaded, and the accept filter's callback points to random memory since
preload_delete_name() unmaps the file on x86 as of r336505.

Add a new ACCEPT_FILTER_DEFINE macro which wraps the accept filter and
module definitions, and ensures that a module version is defined.

PR: 245870
Reported by: Thomas von Dein <freebsd@daemon.de>
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation