convert a bunch of open-coded instances of memdup_user_nul()

A _lot_ of ->write() instances were open-coding it; some are
converted to memdup_user_nul(), a lot more remain...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index e3952e9..fe42b6e 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -657,14 +657,9 @@
 		pr_warn("expected <%d bytes into control\n", USER_BUF_PAGE);
 		return -E2BIG;
 	}
-	tmpbuf = kmalloc(len + 1, GFP_KERNEL);
-	if (!tmpbuf)
-		return -ENOMEM;
-	if (copy_from_user(tmpbuf, ubuf, len)) {
-		kfree(tmpbuf);
-		return -EFAULT;
-	}
-	tmpbuf[len] = '\0';
+	tmpbuf = memdup_user_nul(ubuf, len);
+	if (IS_ERR(tmpbuf))
+		return PTR_ERR(tmpbuf);
 	vpr_info("read %d bytes from userspace\n", (int)len);
 
 	ret = ddebug_exec_queries(tmpbuf, NULL);