p9fs: move NULL check immediately after allocation

Reported by:	Shawn Webb (HardenedBSD)
Reviewed by:	dfr
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45719

sys/dev/virtio/p9fs/virtio_p9fs.c

@@ -332,6 +332,11 @@ vt9p_attach(device_t dev)
 	cv_init(&chan->submit_cv, "Conditional variable for submit queue" );
 	chan->max_nsegs = MAX_SUPPORTED_SGS;
 	chan->vt9p_sglist = sglist_alloc(chan->max_nsegs, M_NOWAIT);
+	if (chan->vt9p_sglist == NULL) {
+		error = ENOMEM;
+		P9_DEBUG(ERROR, "%s: Cannot allocate sglist\n", __func__);
+		goto out;
+	}
 
 	/* Negotiate the features from the host */
 	virtio_set_feature_desc(dev, virtio_9p_feature_desc);
@@ -367,12 +372,6 @@ vt9p_attach(device_t dev)
 	SYSCTL_ADD_STRING(ctx, SYSCTL_CHILDREN(tree), OID_AUTO, "p9fs_mount_tag",
 	    CTLFLAG_RD, chan->mount_tag, 0, "Mount tag");
 
-	if (chan->vt9p_sglist == NULL) {
-		error = ENOMEM;
-		P9_DEBUG(ERROR, "%s: Cannot allocate sglist\n", __func__);
-		goto out;
-	}
-
 	/* We expect one virtqueue, for requests. */
 	error = vt9p_alloc_virtqueue(chan);