do_proc_readlink(): constify path

Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc/base.c

@@ -1761,7 +1761,7 @@ out:
 	return ERR_PTR(error);
 }
 
-static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
+static int do_proc_readlink(const struct path *path, char __user *buffer, int buflen)
 {
 	char *tmp = kmalloc(PATH_MAX, GFP_KERNEL);
 	char *pathname;