Btrfs: fix unzeroed members in fs_devices when creating a fs from seed fs

We forgot to zero some members in fs_devices when we create new fs_devices
from the one of the seed fs. It would cause the problem that we got wrong
chunk profile when allocating chunks. Fix it.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/volumes.c

@@ -1988,6 +1988,9 @@ static int btrfs_prepare_sprout(struct btrfs_root *root)
 	fs_devices->seeding = 0;
 	fs_devices->num_devices = 0;
 	fs_devices->open_devices = 0;
+	fs_devices->missing_devices = 0;
+	fs_devices->num_can_discard = 0;
+	fs_devices->rotating = 0;
 	fs_devices->seed = seed_devices;
 
 	generate_random_uuid(fs_devices->fsid);