Btrfs: close devices on all error paths in open_ctree()

Fix a bug introduced by 7e662854

 where we would leave devices busy on
certain error paths in open_ctree().  fs_info is guaranteed to be
non-NULL now so it's safe to dereference it on all error paths.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>