Btrfs-progs: fix btrfs-convert rollback to check ROOT_BACKREF

Btrfs-progs: fix btrfs-convert rollback to check ROOT_BACKREF

Btrfs has changed to delete subvolume/snapshot asynchronously, which
means that after umount itself, if we've already deleted 'ext2_saved',
rollback can still be completed.

So this adds a check for ROOT_BACKREF before checking ROOT_ITEM since
ROOT_BACKREF is immediately not in the btree after
ioctl(BTRFS_IOC_SNAP_DESTROY) returns.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ updated error messages ]
Signed-off-by: David Sterba <dsterba@suse.com>