Add slab usage summeries to /proc

One of the most common things you want to know when looking at
the slab is how much memory is being used. This information was
available in /proc/spl/kmem/slab but only on a per-slab basis.
This commit adds the following /proc/sys/kernel/spl/kmem/slab*
entries to make total slab usage easily available at a glance.

slab_kmem_total - Total kmem slab size
slab_kmem_avail - Alloc'd kmem slab size
slab_kmem_max   - Max observed kmem slab size
slab_vmem_total - Total vmem slab size
slab_vmem_avail - Alloc'd vmem slab size
slab_vmem_max   - Max observed vmem slab size