IB/hfi1: Prevent a NULL dereference

In the original code, we set "fd->uctxt" to NULL and then dereference it
which will cause an Oops.

Fixes: f2a3bc00a03c ("IB/hfi1: Protect context array set/clear with spinlock")
Cc: <stable@vger.kernel.org> # 4.14.x
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/hfi1/file_ops.c

@@ -763,11 +763,11 @@ static int complete_subctxt(struct hfi1_filedata *fd)
 	}
 
 	if (ret) {
-		hfi1_rcd_put(fd->uctxt);
-		fd->uctxt = NULL;
 		spin_lock_irqsave(&fd->dd->uctxt_lock, flags);
 		__clear_bit(fd->subctxt, fd->uctxt->in_use_ctxts);
 		spin_unlock_irqrestore(&fd->dd->uctxt_lock, flags);
+		hfi1_rcd_put(fd->uctxt);
+		fd->uctxt = NULL;
 	}
 
 	return ret;