Fix ASSERTION(!dsl_pool_sync_context(tx->tx_pool))

Disable the normal reclaim path for the txg_sync thread.  This
ensures the thread will never enter dmu_tx_assign() which can
otherwise occur due to direct reclaim.  If this is allowed to
happen the system can deadlock.  Direct reclaim call path:

  ->shrink_icache_memory->prune_icache->dispose_list->
  clear_inode->zpl_clear_inode->zfs_inactive->dmu_tx_assign

module/zfs/txg.c

@@ -372,6 +372,19 @@ txg_sync_thread(dsl_pool_t *dp)
 	callb_cpr_t cpr;
 	uint64_t start, delta;
 
+#ifdef _KERNEL
+	/*
+	 * Disable the normal reclaim path for the txg_sync thread.  This
+	 * ensures the thread will never enter dmu_tx_assign() which can
+	 * otherwise occur due to direct reclaim.  If this is allowed to
+	 * happen the system can deadlock.  Direct reclaim call path:
+	 *
+	 *   ->shrink_icache_memory->prune_icache->dispose_list->
+	 *   clear_inode->zpl_clear_inode->zfs_inactive->dmu_tx_assign
+	 */
+	current->flags |= PF_MEMALLOC;
+#endif /* _KERNEL */
+
 	txg_thread_enter(tx, &cpr);
 
 	start = delta = 0;