Fix zdb -e and zhack thread_init()

This issue was caused by calling `thread_init()` and `thread_fini()`
multiple times resulting in `kthread_key` being invalid.  To resolve
the issue the explicit calls to `thread_init()` and `thread_fini()`
required by the `zpool` command have been moved in to the command.
Consumers such as `zdb` and `zhack` perform the same initialized
through `kernel_init()` and `kernel_fini()`.

Resolving this issue allows multiple additional test cases to
be enabled.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #4331

lib/libzfs/libzfs_import.c

@@ -1392,6 +1392,7 @@ zpool_find_import_impl(libzfs_handle_t *hdl, importargs_t *iarg)
 			slice->rn_nozpool = B_FALSE;
 			avl_add(&slice_cache, slice);
 		}
+
 		/*
 		 * create a thread pool to do all of this in parallel;
 		 * rn_nozpool is not protected, so this is racy in that
@@ -1401,7 +1402,6 @@ zpool_find_import_impl(libzfs_handle_t *hdl, importargs_t *iarg)
 		 * locks in the kernel, so going beyond this doesn't
 		 * buy us much.
 		 */
-		thread_init();
 		t = taskq_create("z_import", 2 * boot_ncpus, defclsyspri,
 		    2 * boot_ncpus, INT_MAX, TASKQ_PREPOPULATE);
 		for (slice = avl_first(&slice_cache); slice;
@@ -1411,7 +1411,6 @@ zpool_find_import_impl(libzfs_handle_t *hdl, importargs_t *iarg)
 			    TQ_SLEEP);
 		taskq_wait(t);
 		taskq_destroy(t);
-		thread_fini();
 
 		cookie = NULL;
 		while ((slice = avl_destroy_nodes(&slice_cache,