block: fix locking in bdev_del_partition

We need to hold the whole device bd_mutex to protect against
other thread concurrently deleting out partition before we get
to it, and thus causing a use after free.

Fixes: cddae808 ("block: pass a hd_struct to delete_partition")
Reported-by:  <syzbot+6448f3c229bc52b82f69@syzkaller.appspotmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>