LU-4423 lnet: fix return value check in lnet_sock_ioctl()

LU-4423 lnet: fix return value check in lnet_sock_ioctl()

Linux-commit: b3c1afd1fa366ef733a9f16c493a03396d7e4ab5

In case of error, the function sock_alloc_file() 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Change-Id: Ib1e04da4ab2d637a632f0b4e84f38ed6b114e27e
Reviewed-on: http://review.whamcloud.com/15125
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>