Use an rwsem to protect the shrinker list instead of a regular
semaphore.  Modifications to the list are now done under the write lock,
shrink_slab takes the read lock, and access to shrinker->nr becomes racy
(which is no different to how the page lru scanner is implemented).  The
shrinker functions become concurrent.

Previously, having the slab scanner get preempted or scheduling while
holding the semaphore would cause other tasks to skip putting pressure on
the slab.

Also, make shrink_icache_memory return -1 if it can't do anything in order
to hold pressure on this cache and prevent useless looping in shrink_slab.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>