drm/i915: shut up spurious WARN in the gtt fault handler

-ENOSPC can happen if userspace is being simplistic and tries to map a
too big object. To aid further spurious WARN debugging, also print out
the error code.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56017
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1408,8 +1408,10 @@ out:
 		return VM_FAULT_NOPAGE;
 	case -ENOMEM:
 		return VM_FAULT_OOM;
+	case -ENOSPC:
+		return VM_FAULT_SIGBUS;
 	default:
-		WARN_ON_ONCE(ret);
+		WARN_ONCE(ret, "unhandled error in i915_gem_fault: %i\n", ret);
 		return VM_FAULT_SIGBUS;
 	}
 }