We have interesting deadlocks when slab decides to use order-1 allocations for
ext3_inode_cache.  This is because ext3_alloc_inode() needs to perform a
GFP_NOFS 1-order allocation.

Sometimes the 1-order allocation needs to free a huge number of pages (tens of
megabytes) before a 1-order grouping becomes available.  But the GFP_NOFS
allocator cannot free dcache (and hence icache) due to the deadlock problems
identified in shrink_dcache_memory().

So change slab so that it will force 0-order allocations for shrinkable VFS
objects.  We can handle those OK.