ubi: block: fix memleak in ubiblock_create()

If idr_alloc() fails, dev->gd will be put after goto out_cleanup_disk in
ubiblock_create(), but dev->gd has not been assigned yet at this time, and
'gd' will not be put anymore. Fix it by putting 'gd' directly.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>