fsck.gfs2: Retain context for indirect pointers in ->check_metalist
    
    When the pass->check_metalist() functions are called, the pointer is
    thrown away and the function is just called with a block address,
    meaning that ->check_metalist() is unable to fix the pointer itself if
    it deems the block pointed to is garbage.
    
    Add an iptr structure which collects together the inode, the metadata
    block buffer and the offset of the pointer in that buffer. The iptr is
    now passed to ->check_metadata instead of the separate inode and block
    address arguments.
    
    Resolves: rhbz#1487726
    
    Signed-off-by: Andrew Price <anprice@redhat.com>