Implemented zpool sync command

This addition will enable us to sync an open TXG to the main pool
on demand. The functionality is similar to 'sync(2)' but 'zpool sync'
will return when data has hit the main storage instead of potentially
just the ZIL as is the case with the 'sync(2)' cmd.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #6122

include/libzfs.h

@@ -26,6 +26,7 @@
  * Copyright (c) 2013 Steven Hartland. All rights reserved.
  * Copyright (c) 2016, Intel Corporation.
  * Copyright 2016 Nexenta Systems, Inc.
+ * Copyright (c) 2017 Datto Inc.
  */
 
 #ifndef	_LIBZFS_H
@@ -264,6 +265,8 @@ extern int zpool_clear(zpool_handle_t *, const char *, nvlist_t *);
 extern int zpool_reguid(zpool_handle_t *);
 extern int zpool_reopen(zpool_handle_t *);
 
+extern int zpool_sync_one(zpool_handle_t *, void *);
+
 extern int zpool_vdev_online(zpool_handle_t *, const char *, int,
     vdev_state_t *);
 extern int zpool_vdev_offline(zpool_handle_t *, const char *, boolean_t);

include/libzfs_core.h

@@ -21,6 +21,7 @@
 
 /*
  * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2017 Datto Inc.
  */
 
 #ifndef	_LIBZFS_CORE_H
@@ -89,6 +90,8 @@ boolean_t lzc_exists(const char *);
 
 int lzc_rollback(const char *, char *, int);
 
+int lzc_sync(const char *, nvlist_t *, nvlist_t **);
+
 #ifdef	__cplusplus
 }
 #endif

include/sys/fs/zfs.h

@@ -24,6 +24,7 @@
  * Copyright (c) 2011, 2014 by Delphix. All rights reserved.
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
  * Copyright (c) 2013, Joyent, Inc. All rights reserved.
+ * Copyright (c) 2017 Datto Inc.
  */
 
 /* Portions Copyright 2010 Robert Milkowski */
@@ -1020,6 +1021,7 @@ typedef enum zfs_ioc {
 	ZFS_IOC_GET_BOOKMARKS,
 	ZFS_IOC_DESTROY_BOOKMARKS,
 	ZFS_IOC_RECV_NEW,
+	ZFS_IOC_POOL_SYNC,
 
 	/*
 	 * Linux - 3/64 numbers reserved.