Shrinking a huge number of dentries or inodes can hold dcache_lock or
inode_lock for a long time.  Not only does this hold off preemption -
holding those locks basically shuts down the whole VFS.

A neat fix for all such caches is to chunk the work up at the
shrink_slab() level.

I made the chunksize pretty small, for scalability reasons - avoid
holding the lock for too long so another CPU can come in, acquire it
and go off to do some work.