Btrfs: fix missing check before creating a qgroup relation

Step to reproduce:
		mkfs.btrfs <disk>
		mount <disk> <mnt>
		btrfs quota enable <mnt>
		btrfs qgroup assign 0/1 1/1 <mnt>
		umount <mnt>
		btrfs-debug-tree <disk> | grep QGROUP
If we want to add a qgroup relation, we should gurantee that
'src' and 'dst' exist, otherwise, such qgroup relation should
not be allowed to create.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>