Attached patch implements an O_STREAMING file I/O flag which enables
manual drop-behind of pages.

If the file has O_STREAMING set then the user has explicitly said "this
is streaming data, I know I will not revisit this, do not cache
anything".  So we drop pages from the pagecache before our current
index.  We have to fiddle a bit to get writes working since we do
write-behind but the logic is there and it works.

Some numbers.  A simple streaming read to verify the pagecache effects:

        Streaming 1GB Read (avg of many runs, mem=2GB):
        O_STREAMING     Wall time       Change in Page Cache
        Yes             25.58s          0
        No              25.55s          +835MB

Another read with much more VM pressure:

        Streaming 1GB Read (avg of many runs, mem=8M)
        O_STREAMING     Wall time       Change in Page Cache
        Yes             25.76s          0
        No              29.01s          +1MB

And now the kicker:

        Kernel compile (make -j2) and concurrent streaming I/O
        (avg of two runs, mem=128M):
        O_STREAMING     Time to complete Kernel Compile
        Yes             3m27.863s
        No              4m15.818s

This is c/o Andrew Morton.