btrfs: fix null pointer deference at btrfs_sysfs_add_one+0x105

bdev is null when disk has disappeared and mounted with
the degrade option

stack trace
---------
btrfs_sysfs_add_one+0x105/0x1c0 [btrfs]
open_ctree+0x15f3/0x1fe0 [btrfs]
btrfs_mount+0x5db/0x790 [btrfs]
? alloc_pages_current+0xa4/0x160
mount_fs+0x34/0x1b0
vfs_kern_mount+0x62/0xf0
do_mount+0x22e/0xa80
? __get_free_pages+0x9/0x40
? copy_mount_options+0x31/0x170
SyS_mount+0x7e/0xc0
system_call_fastpath+0x16/0x1b
---------

reproducer:
-------
mkfs.btrfs -draid1 -mraid1 /dev/sdc /dev/sdd
(detach a disk)
devmgt detach /dev/sdc [1]
mount -o degrade /dev/sdd /btrfs
-------

[1] github.com/anajain/devmgt.git
Signed-off-by: NAnand Jain <Anand.Jain@oracle.com>
Tested-by: NHidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/sysfs.c

@@ -578,8 +578,14 @@ static int add_device_membership(struct btrfs_fs_info *fs_info)
 		return -ENOMEM;
 
 	list_for_each_entry(dev, &fs_devices->devices, dev_list) {
-		struct hd_struct *disk = dev->bdev->bd_part;
-		struct kobject *disk_kobj = &part_to_dev(disk)->kobj;
+		struct hd_struct *disk;
+		struct kobject *disk_kobj;
+
+		if (!dev->bdev)
+			continue;
+
+		disk = dev->bdev->bd_part;
+		disk_kobj = &part_to_dev(disk)->kobj;
 
 		error = sysfs_create_link(fs_info->device_dir_kobj,
 					  disk_kobj, disk_kobj->name);