Do not upgrade userobj accounting for snapshot dataset

'zfs recv' could disown a living objset without calling
dmu_objset_disown(). This will cause the problem that the objset
would be released while the upgrading thread is still running.

This patch avoids the problem by checking if a dataset is a snapshot
before calling dmu_objset_userobjspace_upgrade().  Snapshots
are immutable and therefore it doesn't make sense to update them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Closes #5295 
Closes #5328

include/sys/dmu_objset.h

@@ -190,6 +190,7 @@ boolean_t dmu_objset_userobjspace_present(objset_t *os);
 static inline boolean_t dmu_objset_userobjspace_upgradable(objset_t *os)
 {
 	return (dmu_objset_type(os) == DMU_OST_ZFS &&
+	    !dmu_objset_is_snapshot(os) &&
 	    dmu_objset_userobjused_enabled(os) &&
 	    !dmu_objset_userobjspace_present(os));
 }