vgaarb: fix "target=default" passing

Commit 77c1ff39 fixed the userspace
pointer dereference, but introduced another bug pointed out by Eugene Teo
in RH bug #564264. Instead of comparing the point we were at in the string,
we instead compared the beginning of the string to "default".
Signed-off-by: Kyle McMartin <kyle@redhat.com>
Reported-by: Eugene Teo <eteo@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/vga/vgaarb.c

@@ -961,7 +961,7 @@ static ssize_t vga_arb_write(struct file *file, const char __user * buf,
 		remaining -= 7;
 		pr_devel("client 0x%p called 'target'\n", priv);
 		/* if target is default */
-		if (!strncmp(kbuf, "default", 7))
+		if (!strncmp(curr_pos, "default", 7))
 			pdev = pci_dev_get(vga_default_device());
 		else {
 			if (!vga_pci_str_to_vars(curr_pos, remaining,