btrfs-progs: qgroup: move btrfs_show_qgroups's error handler to __qgroup_search

btrfs-progs: qgroup: move btrfs_show_qgroups's error handler to __qgroup_search

We have to process the return value of BTRFS_IOC_TREE_SEARCH ioctl in
advance, so that we can distinguish between the two case where quota
is not enabled (ioctl return -ENOENT) and either parent qgroup or child
qgroup does not exist (update_qgroup_relation return -ENOENT). Besides
this, any error in this routine has been reported, so we don't need to
report again in cmd_qgroup_show.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>