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
Add ASSERT0 and VERIFY0 macros

The Illumos code introduced the ASSERT0 and VERIFY0 macros which
are to be used instead of ASSERT3S(x, ==, 0) and VERIFY3S(x, ==, 0).

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Madhav Suresh <madhav.suresh@delphix.com>
Closes #246