Some people want the dentry and inode caches shrink harder, others want them
shrunk more reluctantly.

The patch adds /proc/sys/vm/vfs_cache_pressure, which tunes the vfs cache
versus pagecache scanning pressure.

- at vfs_cache_pressure=0 we don't shrink dcache and icache at all.

- at vfs_cache_pressure=100 there is no change in behaviour.

- at vfs_cache_pressure > 100 we reclaim dentries and inodes harder.


The number of megabytes of slab left after a slocate.cron on my 256MB test
box:

vfs_cache_pressure=100000   33480
vfs_cache_pressure=10000    61996
vfs_cache_pressure=1000     104056
vfs_cache_pressure=200      166340
vfs_cache_pressure=100      190200
vfs_cache_pressure=50       206168

Of course, this just left more directory and inode pagecache behind instead of
vfs cache.  Interestingly, on this machine the entire slocate run fits into
pagecache, but not into VFS caches.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>