ext4: Fix potential truncate BUG due to i_prealloc_list being non-empty

We need to call ext4_discard_reservation() earlier in ext4_truncate(),
to avoid a BUG() in ext4_mb_return_to_preallocation(), which is called
(ultimately) by ext4_free_blocks().  So we must ditch the blocks on
i_prealloc_list before we start freeing the data blocks.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -3494,6 +3494,9 @@ void ext4_truncate(struct inode *inode)
 	 * modify the block allocation tree.
 	 */
 	down_write(&ei->i_data_sem);
+
+	ext4_discard_reservation(inode);
+
 	/*
 	 * The orphan list entry will now protect us from any crash which
 	 * occurs before the truncate completes, so it is now safe to propagate
@@ -3563,8 +3566,6 @@ void ext4_truncate(struct inode *inode)
 		;
 	}
 
-	ext4_discard_reservation(inode);
-
 	up_write(&ei->i_data_sem);
 	inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
 	ext4_mark_inode_dirty(handle, inode);