fs/orangefs: Replace kzalloc + copy_from_user with memdup_user_nul

Replace kzalloc() followed by copy_from_user() with memdup_user_nul() to
simplify and improve orangefs_debug_write(). Allocate only 'count' bytes
instead of the maximum size ORANGEFS_MAX_DEBUG_STRING_LEN, and set 'buf'
to NULL to ensure kfree(buf) still works.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>

Author: toblux

fs/orangefs/orangefs-debugfs.c

@@ -440,14 +440,13 @@ static ssize_t orangefs_debug_write(struct file *file,
 		count = ORANGEFS_MAX_DEBUG_STRING_LEN;
 	}
 
-	buf = kzalloc(ORANGEFS_MAX_DEBUG_STRING_LEN, GFP_KERNEL);
-	if (!buf)
-		goto out;
-
-	if (copy_from_user(buf, ubuf, count - 1)) {
+	buf = memdup_user_nul(ubuf, count - 1);
+	if (IS_ERR(buf)) {
 		gossip_debug(GOSSIP_DEBUGFS_DEBUG,
-			     "%s: copy_from_user failed!\n",
+			     "%s: memdup_user_nul failed!\n",
 			     __func__);
+		rc = PTR_ERR(buf);
+		buf = NULL;
 		goto out;
 	}