Suspected bug in shrink_caches()

[Sasha Pachev <sasha@mysql.com>]

Rik:

I am quite sure there is still a bug in shrink_caches(), at least there was 
one in 2.4.17-rc2. I have not tried later releases, but there is nothing in 
the changelog about the fixes anywhere near that area of the code, so I have 
to assume the problem is still there.

Here is how one can expose the bug:

 - disable kswapd and turn off all swap for simplicity
 - run two applications concurrently, one cache intenstive ( eg find / or 
simply create/initialize/read a large file), and the other RAM-intensive - 
allocate a large block and initialize it

When we get to the point where free memory starts running low, even though we 
may have something like 100 MB of cache, shrink_caches() fails to free up 
enough memory, which triggers the evil oom killer. Obviously, in the above 
situation the correct behaviour is to go on cache diet before considering the 
murders.

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