Merge branch 'pagecache' of https://github.com/gleb-cloudius/osv

"
This is write page cache implemented on top of ARC read cache. On a
read fault pages are initially mapped from ARC directly, but they are
marked as RO in a page table. On a write fault pages are copied into
small write page cache for shared mapping, or into anonymous page for
private mappings.  Pages are removed from write cache and written back
into a file in a FIFO order.

This passes "make check", misc-mmap-big-file modified to do write in
addition to read and "cassandra-stress -d".
"

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>