Fix 'zfs get {user|group}objused@' functionality

Fix a regression accidentally introduced in 1b81ab4 that prevents
'zfs get {user|group}objused@' from correctly reporting the requested
value.

Update "userspace_003_pos.ksh" and "groupspace_003_pos.ksh" to verify
this functionality.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6908