btrfs: fix ulist leaks in error paths of qgroup self tests

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix ulist leaks in error paths of qgroup self tests

In the test_no_shared_qgroup() and test_multiple_refs() qgroup self tests,
if we fail to add the tree ref, remove the extent item or remove the
extent ref, we are returning from the test function without freeing the
"old_roots" ulist that was allocated by the previous calls to
btrfs_find_all_roots(). Fix that by calling ulist_free() before returning.