Fix --enable-debug-kmem-tracking option

This code has gotten something stale and no longer builds cleanly
against modern kernels.  The two issues addressed here are as
follows:

* The hlist_*_rcu interfaces in the kernel have been relatively
  unstable.  Since this isn't performance critical code just use
  the long standing hlist_* variants.

* In older kernels the hash_ptr() function takes a 'void *' but
  in newer kernels it expects a 'const void *'.  To silence the
  compiler warnings about this explicitly cast it to a 'void *'.
  The memset function is a similar case but it always expects
  a 'void *'.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #256
Fix bogus kmem leak warning

Commit 5c7a036 correctly relocated the creation of a taskq
and the registraction of the kmem_cache_shrinker after the
initialization of the kmem tracking code.  However, the
cleanup of these structures was not done before the leak
checks in spl_kmem_fini().  This resulted in an incorrect
'kmem leaked' warning even though there was no actual leak.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#1569