Properly delete csum item in btrfs_truncate_in_trans.

When 'item_end' is equal to 'inode->i_size',   'found_type' is updated
and current item is skipped. This behavior is correct for extent item,
but incorrect for csum item. For example, there is a csum item with
'offset == 0'. When deleting the inode,  'inode->i_size' is set to 0,
so the csum item isn't deleted.

Signed-off-by: Chris Mason <chris.mason@oracle.com>