xfrm: fix ptr_ret.cocci warnings

net/xfrm/xfrm_interface.c:692:1-3: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Fixes: 44e2b838 ("xfrm: Return detailed errors from xfrmi_newlink")
CC: Benedict Wong <benedictwong@google.com>
Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>