btrfs: tests: fix root tree leak in btrfs_test_qgroups()

btrfs: tests: fix root tree leak in btrfs_test_qgroups()

If btrfs_insert_fs_root() fails, the tmp_root allocated by
btrfs_alloc_dummy_root() is leaked because its initial reference count
is not decremented.

Fix this by calling btrfs_put_root() unconditionally after
btrfs_insert_fs_root(). This ensures the local reference is always
dropped.

Also fix a copy-paste error in the error message where the subvolume
root insertion failure was incorrectly logged as "fs root".

Co-developed-by: Jianhao Xu <jianhao.xu@seu.edu.cn>
Signed-off-by: Jianhao Xu <jianhao.xu@seu.edu.cn>
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>