UBIFS: make debugfs files non-seekable

Turn the debufs files UBIFS maintains into non-seekable. Indeed, none
of them is supposed to be seek'ed.

Do this by making the '.lseek()' handler to be 'no_llseek()' and by
using 'nonseekable_open()' in the '.open()' operation.

This does mean an API break but this debugging API is only used by a couple
of test scripts which do not rely in the 'llseek()' operation.
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/debug.c

@@ -2784,7 +2784,7 @@ void dbg_debugfs_exit(void)
 static int open_debugfs_file(struct inode *inode, struct file *file)
 {
 	file->private_data = inode->i_private;
-	return 0;
+	return nonseekable_open(inode, file);
 }
 
 static ssize_t write_debugfs_file(struct file *file, const char __user *buf,
@@ -2814,7 +2814,7 @@ static const struct file_operations dfs_fops = {
 	.open = open_debugfs_file,
 	.write = write_debugfs_file,
 	.owner = THIS_MODULE,
-	.llseek = default_llseek,
+	.llseek = no_llseek,
 };
 
 /**