LU-4423 obdclass: fix return value check in capa_hmac()

LU-4423 obdclass: fix return value check in capa_hmac()

In case of error, the function crypto_alloc_hash() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Change-Id: I4889387752d1eb5400649cd5f4da172d64c054e2
Reviewed-on: http://review.whamcloud.com/8681
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>