LU-4524 ldlm: improve ldlm_lock_create() return value

LU-4524 ldlm: improve ldlm_lock_create() return value

ldlm_lock_create() and ldlm_resource_get() always return NULL as
error reporting and "NULL" is interpretted as ENOMEM incorrectly
sometimes.
This patch fixes this problem by using ERR_PTR() rather than NULL.

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: Ifa2e9494406617c9d29c0463cd9fcffba36cc5ae
Reviewed-on: http://review.whamcloud.com/9004
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>