Use default slab types

We should not override the default memory type of the kmem cache.  This
was done previously to force certain objects which were slightly over
object size limit cut off in to KMC_KMEM caches for better performance.

The zfsonlinux/spl#356 patch slightly increases the default cut off
from 511 bytes 1024 bytes for x86_64.  This means there is long longer
a need to override the default for the caches.  And since the default
values are now being used the new spl_kmem_cache_slab_limit and
spl_kmem_cache_kmem_limit tunables will apply to all kmem caches.

The following is a list of caches that will be impacted:

                   <object size>   <forced type>   <default type>
dnode_t            936 bytes       KMC_KMEM        KMC_KMEM
zio_cache          1104 bytes      *KMC_KMEM       *KMC_VMEM
zio_link_cache     48 bytes        KMC_KMEM        KMC_KMEM
zio_vdev_cache     131088 bytes    KMC_VMEM        KMC_VMEM
zio_buf_512        512 bytes       KMC_KMEM        KMC_KMEM
zio_data_buf_512   512 bytes       KMC_KMEM        KMC_KMEM
zio_buf_1024       1024 bytes      KMC_KMEM        KMC_KMEM
zio_data_buf_1024  1024 bytes      +KMC_VMEM       KMC_KMEM

* Cache memory type will change from KMC_KMEM to KMC_VMEM.
+ Cache memory type will change from KMC_VMEM to KMC_KMEM.

This patch removes another slight point of divergence between ZoL
and Illumos.

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