[SCSI] enclosure: fix WARN_ON in dual path device removing

Bug report from: wenxiong@linux.vnet.ibm.com

The issue is happened in dual controller configuration. We got the
sysfs warnings when rmmod the ipr module.

enclosure_unregister() in drivers/msic/enclosure.c, call device_unregister()
for each componment deivce, device_unregister() ->device_del()->kobject_del()
->sysfs_remove_dir(). In sysfs_remove_dir(), set kobj->sd = NULL.

For each componment device,
enclosure_component_release()->enclosure_remove_links()->sysfs_remove_link()
in which checking kobj->sd again, it has been set as NULL when doing
device_unregister. So we saw all these sysfs WARNING.

Tested-by:  <wenxiong@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>