Currently there is no way to find out the effective object size of a slab
cache.  XFS has lots of IRIX-derived code that want to do zalloc() style
allocations on zones (which are implemented as slab caches in XFS/Linux)
and thus needs to know about it.  There are three ways do implement it:

a) implement kmem_cache_zalloc
b) make the xfs zone a struct of kmem_cache_t and a size variable
c) implement kmem_cache_size

The current XFS tree does a) but I absolutely don't like it as encourages
people to use kmem_cache_zalloc for new code instead of thinking about how
to utilize slab object reuse.  b) would be easy, but I guess kmem_cache_size
is usefull enough to get into the kernel.  Here's the patch: