Fix --enable-debug-kmem-tracking option

Fix --enable-debug-kmem-tracking option

Re-order initialization in spl_kmem_init to allow for kmem tracing
to work.  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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #243