Btrfs: fix wrong search path initialization before searching tree root

To search tree root without transaction protection, we should neither search commit
root nor skip locking here, fix it.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>