staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4524
Reviewed-on: http://review.whamcloud.com/9004


Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>