dm ioctl: fix hang in early create error condition

The dm_early_create() function (which deals with "dm-mod.create=" kernel
command line option) calls dm_hash_insert() who gets an extra reference
to the md object.

In case of failure, this reference wasn't being released, causing
dm_destroy() to hang, thus hanging the whole boot process.

Fix this by calling __hash_remove() in the error path.

Fixes: 6bbc923d

 ("dm: add support to directly boot to a mapped device")
Cc: stable@vger.kernel.org
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>