rbd: don't leak parent_spec in rbd_dev_probe_parent()

commit 1f2c6651f69c14d0d3a9cfbda44ea101b02160ba upstream.

Currently we leak parent_spec and trigger a "parent reference
underflow" warning if rbd_dev_create() in rbd_dev_probe_parent() fails.
The problem is we take the !parent out_err branch and that only drops
refcounts; parent_spec that would've been freed had we called
rbd_dev_unparent() remains and triggers rbd_warn() in
rbd_dev_parent_put() - at that point we have parent_spec != NULL and
parent_ref == 0, so counter ends up being -1 after the decrement.

Redo rbd_dev_probe_parent() to fix this.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
[idryomov@gmail.com: backport to < 4.2: rbd_dev->opts]
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>