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.  Additionally
the slab size is rounded up to a page which is what kv_alloc()
expects.  The kv_alloc() code will minimally allocate a page,
in the KMC_OFFSLAB case this could be reduced.

The basic regression tests kmem:slab_small, kmem:slab_large,
and kmem:slab_align regression were updated to test KMC_OFFSLAB.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>