Initialize dbu_tqent in dmu_buf_init_user()

The dbu_evict_taskq added in 0c66c32 is only invoked via
taskq_dispatch_ent(). In these cases, ZoL's implementation of taskqs
requires the entries to be initialized first with taskq_init_ent() in
order that, among other things, the embedded spinlock is initialized
properly.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3419

include/sys/dmu.h

@@ -558,6 +558,7 @@ dmu_buf_init_user(dmu_buf_user_t *dbu, dmu_buf_evict_func_t *evict_func,
 	ASSERT(dbu->dbu_evict_func == NULL);
 	ASSERT(evict_func != NULL);
 	dbu->dbu_evict_func = evict_func;
+	taskq_init_ent(&dbu->dbu_tqent);
 #ifdef ZFS_DEBUG
 	dbu->dbu_clear_on_evict_dbufp = clear_on_evict_dbufp;
 #endif