Btrfs: sysfs: fix, fs_info kobject_unregister has init_completion() twice

kobject_unregister is to handle the release of the kobject,
its completion init is being called in btrfs_sysfs_add_one(),
so we don't have to do the same in the open_ctree() again.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>