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/fs/dlm/user.c b/fs/dlm/user.c
index 173b387..1925d6d 100644
--- a/fs/dlm/user.c
+++ b/fs/dlm/user.c
@@ -515,14 +515,9 @@
 	if (count > sizeof(struct dlm_write_request) + DLM_RESNAME_MAXLEN)
 		return -EINVAL;
 
-	kbuf = kzalloc(count + 1, GFP_NOFS);
-	if (!kbuf)
-		return -ENOMEM;
-
-	if (copy_from_user(kbuf, buf, count)) {
-		error = -EFAULT;
-		goto out_free;
-	}
+	kbuf = memdup_user_nul(buf, count);
+	if (!IS_ERR(kbuf))
+		return PTR_ERR(kbuf);
 
 	if (check_version(kbuf)) {
 		error = -EBADE;