LU-1876 hsm: bugfix about layout lock on the client

LU-1876 hsm: bugfix about layout lock on the client

The following issues are fixed in this patch:
* deadlock at add_lsmref
* lov_delete_raid0 should wait for refcount of lsm to be zero
* handle empty layout at lov layer since layout can be changed
  anytime so original assumption of skipping lov layer when lsm is
  NULL is no longer correct
* other fixes

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ie7cd744b188f4d28fdbebda8870259b931328d00
Reviewed-on: http://review.whamcloud.com/4416
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>