Don't remove the virtual-to-physical mapping when an sf_buf is freed.

Instead, allow the mapping to persist, but add the sf_buf to a free list.
If a later sendfile(2) or zero-copy send resends the same physical page,
perhaps with the same or different contents, then the mapping overhead is
avoided and the sf_buf is simply removed from the free list.

In other words, the i386 sf_buf implementation now behaves as a cache of
virtual-to-physical translations using an LRU replacement policy on
inactive sf_bufs.  This is similar in concept to a part of
http://www.cs.princeton.edu/~yruan/debox/ patch, but much simpler in
implementation.  Note: none of this is required on alpha, amd64, or ia64.
They now use their direct virtual-to-physical mapping to avoid any
emphemeral mapping overheads in their sf_buf implementations.

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