Relax vflush FORCECLOSE special vnode to only protect devfs vnodes

[cse_cem_gmail_com]

Summary

The VNASSERT in vflush() FORCECLOSE case is trying to panic early to prevent errors from yanking devices out from under filesystems. After discussion with kib, we probably only care about special vnodes on devfs.

Sponsored by: EMC / Isilon Storage Division

[cse_cem_gmail_com]

Per discussion on IRC. Move extern declaration into sys/vnode.h so we don't need to import devfs.h in vfs_subr.c.

Drop static declaration of vfs_specops entirely; drop static on actual definition.

[cse_cem_gmail_com]

Committed in r279362.