block: genhd: fix double kfree() in __alloc_disk_node()

syzbot is reporting use-after-free read at bdev_free_inode() [1], for
kfree() from __alloc_disk_node() is called before bdev_free_inode()
(which is called after RCU grace period) reads bdev->bd_disk and calls
kfree(bdev->bd_disk).

Fix use-after-free read followed by double kfree() problem
by making sure that bdev->bd_disk is NULL when calling iput().

Link: https://syzkaller.appspot.com/bug?extid=8281086e8a6fbfbd952a

 [1]
Reported-by: syzbot <syzbot+8281086e8a6fbfbd952a@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/e6dd13c5-8db0-4392-6e78-a42ee5d2a1c4@i-love.sakura.ne.jp


Signed-off-by: Jens Axboe <axboe@kernel.dk>