libgfs2: Fix pointer cast byte order issue
    
    lgfs2_field_assign() currently uses pointer casting to achieve generic
    integer assignment based on the width of the field, but this is broken
    as a uin32_t field can be assigned the value from the high bytes of the
    uint64_t value, for instance. To fix this, store the value into a
    uint64_t before casting to the narrower types.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>