from the existing modevent / MOD_UNLOAD handler, and register div_destroy()
in protosw as per-vnet .pr_destroy() handler for options VIMAGE builds. In
nooptions VIMAGE builds, div_destroy() will be invoked from the modevent
handler, resulting in effectively identical operation as it was prior this
change. div_destroy() also tears down hashtables used by ipdivert, which
were previously left behind on ipdivert kldunloads.
For options VIMAGE builds only, temporarily disable kldunloading of ipdivert,
because without introducing additional locking logic it is impossible to
atomically check whether all ipdivert instances in all vnets are idle, and
proceed with cleanup without opening a race window for a vnet to open an
ipdivert socket while ipdivert tear-down is in progress.
While here, staticize div_init(), because it is not used outside of
ip_divert.c.
In cooperation with: julian
Approved by: re (rwatson), julian (mentor)
MFC after: 3 days
2b73aacaf9