sanlock: fix checksum endian handling
    
    checksums must be computed while the data is in the
    ondisk format (little endian).
    
    When writing a structure:
    - byte swap the structure data from host format to ondisk format
    - compute the host format checksum value from the ondisk data
    - byte swap the checksum value from host format to ondisk format
    - write the ondisk format checksum into the structure
    
    When reading a structure:
    - compute the host format checksum from the ondisk data
    - byte swap the structure data from ondisk format to host format
    - verify the computed host format checksum matches the
      checksum in the host format structure
    
    Signed-off-by: David Teigland <teigland@redhat.com>