Shrink struct vnode by 124 bytes

[Konstantin Belousov]

Hello arch@,

I wrote a series of patches that shrink the struct vnode by 124 bytes,
that is, the sizeof(struct bufobj), for filesystems that do not use
buffer cache.  For them, embedded struct bufobj is a wasted memory, which
is, with typical 10,000,000 or more vnodes on today busy fileserver, is
quite significant kernel memory save.

See the review https://reviews.freebsd.org/D28856 giving the cumulative
diff over the branch, and individual commits with more detailed
explanations for each change at
https://kib.kiev.ua/git/gitweb.cgi?p=deviant3.git;a=shortlog;h=refs/heads/vn_clusterw

Please review and comment.