btrfs: delayed-ref: double free in btrfs_add_delayed_tree_ref()

There is a cut and paste error so instead of freeing "head_ref", we free
"ref" twice.

Fixes: 3368d001 ('btrfs: qgroup: Record possible quota-related extent for qgroup.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>