new helper: uaccess_kernel()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/block/bsg.c b/block/bsg.c
index 74835db..69ccb78 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -650,7 +650,7 @@ bsg_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
 
 	dprintk("%s: write %zd bytes\n", bd->name, count);
 
-	if (unlikely(segment_eq(get_fs(), KERNEL_DS)))
+	if (unlikely(uaccess_kernel()))
 		return -EINVAL;
 
 	bsg_set_block(bd, file);