Use the head of compound pages
    
    In e3e6a3b76804d I had modified page_get_inode() to check for tail
    mappings specifically. A problem that was encountered on Ubuntu was that
    tail mappings might use a seemingly bogus poison offset of 0xffffffff
    instead of TAIL_MAPPING. Fix this by always operating on the compound
    head instead of using a tail.
    
    Resolves: #184
    See also: #40