sg_logs: Fix parameter lengths in show_data_compression_page

[patrickdowling]

For the parameters in show_data_compression_page the length pl is initialized from bp[3] and checked for 0 || > 8, but the value passed to sg_get_unaligned_be is off-by-four. So all values in this page were effectively truncated to 0.

Output from sg_logs /dev/sg0 -p 0x1b before

Data compression log page  (ssc-4) [0x1b]
  Read compression ratio x100: 0
  Write compression ratio x100: 0
  Megabytes transferred to server: 0
  Bytes transferred to server: 0
  Megabytes read from tape: 0
  Bytes read from tape: 0
  Megabytes transferred from server: 0
  Bytes transferred from server: 0
  Megabytes written to tape: 0
  Bytes written to tape: 0
  Data compression enabled: 0
  unknown Parameter code = 0xabcd, contents in hex:
    00 00 00 00

and after fix

Data compression log page  (ssc-4) [0x1b]
  Read compression ratio x100: 42
  Write compression ratio x100: 84
  Megabytes transferred to server: 1234
  Bytes transferred to server: 4567
  Megabytes read from tape: 0
  Bytes read from tape: 0
  Megabytes transferred from server: 0
  Bytes transferred from server: 0
  Megabytes written to tape: 0
  Bytes written to tape: 0
  Data compression enabled: 1
  unknown Parameter code = 0xabcd, contents in hex:
    00 00 00 00 ff 00 ff 00

I'm still playing around with emulation code and not an actual device, yes, so those are dummy values.
The unknown parameter was injected for the 8 byte length, the values in the page covered 1/2/4 already. I think the data I'm generating is correct, but :)

(Similar-ish code in that file sometimes prefers pl + 4 but made for a more invasive change).