lzop: code shrink by using header_t matching on-disk layout

lzop: code shrink by using header_t matching on-disk layout

function                                             old     new   delta
add_bytes_to_chksum                                   37      47     +10
lzo_decompress                                       524     532      +8
init_chksum                                           14      21      +7
chksum_getresult                                      13      17      +4
f_read                                                33      28      -5
f_write8                                              20       -     -20
f_write32                                             22       -     -22
f_write16                                             25       -     -25
f_write                                               36       -     -36
do_lzo_compress                                      328     232     -96
do_lzo_decompress                                    526     411    -115
------------------------------------------------------------------------------
(add/remove: 0/4 grow/shrink: 4/3 up/down: 29/-319)          Total: -290 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>