Btrfs: add a comment explaining what btrfs_cont_expand does

Everytime I have to deal with btrfs_cont_expand I stare at it for 20 minutes
trying to remember what exactly it does and why the hell we need it.  So add a
comment to save future-Josef some time.  Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/inode.c

@@ -3557,6 +3557,12 @@ static int btrfs_truncate_page(struct address_space *mapping, loff_t from)
 	return ret;
 }
 
+/*
+ * This function puts in dummy file extents for the area we're creating a hole
+ * for.  So if we are truncating this file to a larger size we need to insert
+ * these file extents so that btrfs_get_extent will return a EXTENT_MAP_HOLE for
+ * the range between oldsize and size
+ */
 int btrfs_cont_expand(struct inode *inode, loff_t oldsize, loff_t size)
 {
 	struct btrfs_trans_handle *trans;