Fix KMC_OFFSLAB type caches

Because spl_slab_size() was always returning -ENOSPC for caches of
type KMC_OFFSLAB the cache could never be created.  Update the
function to return success and add KMC_OFFSLAB caches to the basic
kmem:slab_small, kmem:slab_large, kmem:slab_align regression tests.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>