Re-order initialization in spl_kmem_init

The spl_kmem_init function calls taskq_create prior to initializing
the tracking (calling spl_kmem_init_tracking).  Since taskq_create uses
kmem_alloc, NULL dereferences occur because the global kmem_list hasn't
had its next & prev pointers initialized yet.

This commit moves the calls to spl_kmem_init_tracking earlier in the
spl_kmem_init function in order that the subsequent kmem_alloc calls
(by taskq_create) work properly.