For 32-bit machines rollback the default number of vnode pager pbufs
back to the lever before r343030. For 64-bit machines reduce it slightly,
too. Together with r343030 I bumped the limit up to the value we use at
Netflix to serve 100 Gbit/s of sendfile traffic, and it probably isn't a
good default.

Provide a loader tunable to change vnode pager pbufs count. Document it.