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>

include/zfeature_common.h

@@ -54,6 +54,7 @@ typedef enum spa_feature {
 	SPA_FEATURE_SHA512,
 	SPA_FEATURE_SKEIN,
 	SPA_FEATURE_EDONR,
+	SPA_FEATURE_USEROBJ_ACCOUNTING,
 	SPA_FEATURES
 } spa_feature_t;