IB/core: Avoid a potential OOPs for an unused optional parameter

The ev_file is an optional parameter for CQ creation. If the parameter
is not passed, the ev_file pointer will be NULL.  Using that pointer
to set the cq_context will result in an OOPs.

Verify that ev_file is not NULL before using.

Cc: <stable@vger.kernel.org> # 4.14.x
Fixes: 9ee79fce

 ("IB/core: Add completion queue (cq) object actions")
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>