Btrfs: remove obsolete FIXMEs in qgroup ioctls

These FIXMEs were already addressed in 2013. All functions check for
qgroup existence:

* btrfs_add_qgroup_relation
* btrfs_ioctl_qgroup_create
* btrfs_limit_qgroup
* btrfs_del_qgroup_relation

Signed-off-by: Daichou <tommy0705c@gmail.com>
[ enhance and reformat changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -4897,7 +4897,6 @@ static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
 		goto out;
 	}
 
-	/* FIXME: check if the IDs really exist */
 	if (sa->assign) {
 		ret = btrfs_add_qgroup_relation(trans, fs_info,
 						sa->src, sa->dst);
@@ -4956,7 +4955,6 @@ static long btrfs_ioctl_qgroup_create(struct file *file, void __user *arg)
 		goto out;
 	}
 
-	/* FIXME: check if the IDs really exist */
 	if (sa->create) {
 		ret = btrfs_create_qgroup(trans, fs_info, sa->qgroupid);
 	} else {
@@ -5010,7 +5008,6 @@ static long btrfs_ioctl_qgroup_limit(struct file *file, void __user *arg)
 		qgroupid = root->root_key.objectid;
 	}
 
-	/* FIXME: check if the IDs really exist */
 	ret = btrfs_limit_qgroup(trans, fs_info, qgroupid, &sa->lim);
 
 	err = btrfs_end_transaction(trans);