LU-607 Avoid race between ldlm_pools_shrink and ldlm_namespace_free

LU-607 Avoid race between ldlm_pools_shrink and ldlm_namespace_free

ldlm_namespace_free removes namespace from list and free memory
without checking namespace's refcount while ldlm_pools_shrink might
get namespace from the list and start ldlm_pool_shrink() for it.

Bugzila: 24419
Author: Vladimir Saveliev <vladimir.saveliev@oracle.com>
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I757204ad119927cacdb73db66f5e40ace50eeea8
Reviewed-on: http://review.whamcloud.com/1273
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann@whamcloud.com>