Add support for user/group dnode accounting & quota

This patch tracks dnode usage for each user/group in the
DMU_USER/GROUPUSED_OBJECT ZAPs. ZAP entries dedicated to dnode
accounting have the key prefixed with "obj-" followed by the UID/GID
in string format (as done for the block accounting).
A new SPA feature has been added for dnode accounting as well as
a new ZPL version. The SPA feature must be enabled in the pool
before upgrading the zfs filesystem. During the zfs version upgrade,
a "quotacheck" will be executed by marking all dnode as dirty.

ZoL-bug-id: https://github.com/zfsonlinux/zfs/issues/3500

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Johann Lombardi <johann.lombardi@intel.com>

module/zcommon/zfs_prop.c

@@ -52,7 +52,11 @@ const char *zfs_userquota_prop_prefixes[] = {
 	"userused@",
 	"userquota@",
 	"groupused@",
-	"groupquota@"
+	"groupquota@",
+	"userobjused@",
+	"userobjquota@",
+	"groupobjused@",
+	"groupobjquota@"
 };
 
 zprop_desc_t *